Optimize LDAP group sync operations

Separate the LDAP group sync process from the regular LDAP access checks and optimize. 

So far, this is a somewhat working PoC that splits the group sync operation to a new worker. It updates all members for a group as it iterates, instead of looping through users and updating groups that way. I am adding lots of logging because this would have been extremely helpful in the past.

There are lots of things still broken, or not considered. However, at least group members are added and updated when I use it in my idyllic dev environment 😃 

- [x] Sync groups
- [x] Sync admins
- [x] Make it work with all type of LDAP groups (with member, member_uid, etc. attributes)
- [x] Update tests
- [ ] Document
- [x] Add scheduled job? (and associated config)

Do these in another merge request

- [ ] Answer: Do we still need some sort of sync on user sign in?
- [ ] Answer: Should sync time be configurable?
- [ ] Answer: Should the group button to 'Reset cache' be changed to 'Sync now'?

Can the last 3 questions be addressed in a subsequent MR?
Are there other things we're not considering yet?

See merge request !229

Fix an issue with wiki in search results when using ES

These changes are a port of gitlab-org/gitlab-ce!3138 to EE for Elastisearch.

This can be safely merged before the next CE->EE `master` merge.

Fixes #396.

See merge request !286

[ci skip]

Don't render Linked LDAP groups forms when LDAP is disabled

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/119

See merge request !283

Email on push inline diff

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3087 and https://gitlab.com/gitlab-org/gitlab-ce/issues/12821

I will backport 16745c1bb386443d8ea3395d23048b1d40ea0b56 and 92c2effbb334246bfed15a0fd78a58f1cfc8be34 to CE.

In a regular browser:

![Screen_Shot_2016-01-28_at_18.11.36](/uploads/662106793138493165e532ebe575b5f1/Screen_Shot_2016-01-28_at_18.11.36.png)

In Gmail (in Chrome):

![chromegmailnew-vertical-allowed-1366-1](/uploads/7887d702577a16056ba8585220d50e04/chromegmailnew-vertical-allowed-1366-1.png)

Gmail doesn't like monospace fonts, apparently, but that's acceptable.

See merge request !151

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/119

Update gitlab-shell version and doc to 2.6.12



See merge request !280

Disable githoooks for git annex commits

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/40

<s>This fix checks if the commit `SHA` is also in one of the synced `git-annex` branches, which means that it should safely skip git hooks. </s> This may not work with multiple commits to the synced branch

These `git-annex` commits also consistently have a message starting with git-annex blah... Using this, as probability of false positives is very low.

See merge request !272

Add confidential issues

Closes gitlab-org/gitlab-ce#3678

Tasks:

- [X] Add `confidential` flag to `issues` table
- [X] Allow user to mark/unmark an issue as confidential
- [X] Restrict access to confidential issues for non-members/author/assignee
  - [X] Issues list
  - [X] Issue details
  - [X] API
  - [X] Search results when Elasticsearch is disabled
  - [X] Search results when Elasticsearch is enabled, and search by term
  - [x] Search results when Elasticsearch is enabled, and search by iid
- [X] Remove references for confidential issues
  - [X] Issue/MR Description
  - [X] Notes
  - [X] Autocomplete
- [x] Milestone overview
  - [x] Hide confidential issues for non-members/author/assignee
  - [x] Does not count confidential issues for non-members/author/assignee
- [X] Add a lock icon to confidential issues
  - [X] Issues list
  - [X] Issue details
  - [X] Search results
- [x] Activity Feed
  - [x] Hide confidential issues for non-members/author/assignee

Screenshots:

* New issue (1):

![new-issue-1](/uploads/b07f8e72cb2183492c142fdeba7ad8a1/new-issue-1.png)

* New issue (2):

![new-issue-2](/uploads/ac3d6ed96d1e5ab95b076bc09c829b3e/new-issue-2.png)

* Issues:

![issues](/uploads/2c891fbe536962a1501723b4cb4681a3/issues.png)

* Issue:

![issue](/uploads/5b11db32ea618c590fc378f21589dd0c/issue.png)

* Search:

![search](/uploads/88591dfc794d7bce097c72d286c87610/search.png)

* Milestone:

![milestone](/uploads/c26b48a1786514b3a3d66026053e9277/milestone.png)

See merge request !227

ES: Index recreation



See merge request !264