Hide SSH repository mirroring

## What does this MR do?
Removes reference to `ssh://` URLs from the repository mirror settings page

## Are there points in the code the reviewer needs to double check?

- [ ] Is the screenshot now used in the documentation an appropriate resolution?
- [ ] Do the remaining screenshots need re-doing to match?

Repository mirroring is also referenced from the 'Import project by URL' functionality, where much of the text of `_instructions.html.haml` is duplicated in a separate partial. It doesn't mention `ssh://`, though.

## Why was this MR needed?

Mirroring repositories over SSH needs more work before it's generally useful.

Currently, these URLs are interpreted with the implicit context of the SSH client configuration for the GitLab installation's `git` user, particularly `known_hosts` and any SSH private keys it has access to. Unless the user manually alters these details  - especially `known_hosts` - then `ssh://` URLs just don't work. 

We don't wish to support and document manual SSH configuration management, and the naive approach (a single `id_rsa` file used when mirroring all repositories) is insecure - especially for GitLab.com or similar deployments.

Future MRs will re-add documentation for `ssh://` URLs for both password and public-key authentication once `known_hosts` and unique SSH keys for repository mirroring are handled automatically.

Disabling support for `ssh://` URLs entirely would break existing customer setups.

## What are the relevant issue numbers?

#98 #621

## Screenshots (if relevant)

![Screen_Shot_2016-08-02_at_10.58.23](/uploads/940bce89b6e8cc109f5df766cd287c7d/Screen_Shot_2016-08-02_at_10.58.23.png)

See merge request !608

Use a full-width, HiDPI screenshot of repository mirroring to more closely match existing screenshots

Move markdown doc to the right location



See merge request !612

[ci skip]

Fix EE code after gitlab_git update from CE

Closes #842

See merge request !610

fix runner install link

## What does this MR do?

Fix link to GitLab CI Runner installation

## Why was this MR needed?

Link is broken

See merge request !5612

Revert "md5 and utf_encode js libraries"

## What does this MR do?

Remove two unused JavaScript libraries (`md5.js` and `utf8_encode.js`).

## Why was this MR needed?

Those libraries were not used anymore:

```
$ find app/ lib/ -name '*.js' -exec grep --perl-regexp --with-filename '(md5|utf8_encode)' {} \;
app/assets/javascripts/lib/utils/utf8_encode.js:function utf8_encode (argString) {
app/assets/javascripts/lib/utils/utf8_encode.js:  // *     example 1: utf8_encode('Kevin van Zonneveld');
app/assets/javascripts/lib/utils/md5.js:function md5 (str) {
app/assets/javascripts/lib/utils/md5.js:  // -    depends on: utf8_encode
app/assets/javascripts/lib/utils/md5.js:  // *     example 1: md5('Kevin van Zonneveld');
app/assets/javascripts/lib/utils/md5.js:  str = this.utf8_encode(str);
```


See merge request !5613

Instrument the Repository class

## What does this MR do?

This MR instruments the Repository class.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

This class wasn't instrumented.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5621

Document wiki linking behavior

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4372

See merge request !609

Since this isn't an ActiveRecord::Base descendant it wasn't
instrumented.

Add help document describing intended wiki linking behavior

Related to #16568
Closes #18019

- This documents the _intended_ behaviour of linking to wiki pages/files.

## TODO

- [x] Respond to @rymai's comments
    - [x] Use `succeed`
    - [x] Shouldn't this be <wiki_root>/miscellaneous.md here?
    - [x] what happens for file links that starts with a `/`?

See merge request !4372

Always compare with FETCH_HEAD in downtime_check

See merge request !5619

fix TODO comment

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20376

After some testing, I've found out this wasn't needed and attachments seemed to work fine for import/export between instances (omnibus & source).

See merge request !5617

Only use RequestStore in ProjectTeam#max_member_access_for_users if it is active

See merge request !5604

This ensures this CI step works properly even when doing a shallow
clone.

ES common index

This will allow as to use parent/child relationship in ES. Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/375

- [x] Fix settings overlapping 
- [x] Specs
- [x] Test everything
- [x] Change rake tasks
- [x] Change index settings for all models
- [x] Update the install doc
- [x] Update the update doc (I created https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5545)
- [x] Prepare text for release blog post


Proposed addition to releasepost:

We totally changed the structure of Elasticsearch  index this is why we need to remove whole index and build new one. To be able to use parent/child relationship in ES we decided to move everything in a single index. This solution has some drawback  - if we decide to change the type of some existing field in ES we will need to rebuild whole index for every entity. So it makes it harder to support but we expect to have more performance advantages with parent/child relationships. 

**Migration**

Remove old indexes

```
curl -XDELETE 'http://localhost:9200/_all/'
```

Build new indexes as described in [Elasticsearch integration](../integration/elasticsearch.md#add-gitlabs-data-to-the-elasticsearch-index)



See merge request !598

Add support for relative links starting with ./ or / to RelativeLinkFilter

## What does this MR do?

- Allow explicit relative links in Markdown (starting with `./`).
- Allow absolute links (relative to the branch's root directory) in Markdown (starting with `/`).

## Why was this MR needed?

Those link types were not supported before.

## What are the relevant issue numbers?

fixes #19028

See merge request !5586

This reverts commit 1bba46d6.

Update installation guide for 8.11

[ci skip]

See merge request !5609

Add migration guide for 8.11

[ci skip]

See merge request !606