Fix bug where manually merged branches in a MR would end up with an empty diff

This bug manifested in 8.1 with the refactoring of `RefreshService`. Here's what happens:

1. User create a new branch `foo`.
2. User creates a merge request for `foo`.
3. User merges `foo` into `master` by hand.
4. `RefreshService` reloads the merge request. Since `master` is equivalent to `foo`, this results in an empty diff.
5. `RefreshService` then closes the MR.

This wasn't an issue when you use the normal "Accept Merge Request" flow because the act of clicking the button closes the merge request, so step 4 never happens.

Closes #3314

See merge request !1758

Enable shared runners for all new projects

This adds Admin Setting option to enable shared runners for all new projects, and by default it is set to true.

/cc @dzaporozhets @sytses 


See merge request !1738

Render same markdown hint for issue, merge request, wiki and comment forms

Fixes internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2595Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1763

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixed markdown issue in PROCESS.md

Hide tab-bar in login-box when only one tabs is shown.

Small UI improvements to merge request page

Make gray block under tabs identical height. Also same height have blocks on pages like `Groups` etc. So this improvement is targeted for consistency in UI too
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes internal issues 

* https://dev.gitlab.org/gitlab/gitlabhq/issues/2591
* https://dev.gitlab.org/gitlab/gitlabhq/issues/2592

cc @DouweM 


See merge request !1759

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

fix line order in installation docs

ugly outlines removed form sidebar

form-control class added to select box

Only have one link in emails to make clicking it easier.



See merge request !1937

Closes #3314

Add CI permissions and api links to README



See merge request !1756

Make migrations reversible



See merge request !1753

Add some polish to CI integration

Check the list of commits to see what I changed.

cc @dzaporozhets

@rspeicher I'd love to get this into 8.1, since it's mostly refactoring/tweaking of stuff that was _introduced_ in 8.1.

Fixes #2645.

See merge request !1641

- Add page titles to CI settings.
- Fix CI admin navigation.
- Remove duplicated scope.
- Use monospace font for commit sha.
- Add page title and header title to build page.
- Proper authorization for cancel/retry builds.
- Use gitlab pagination theme for builds and group members.
- Don't paginate builds widget on build page.
- Add badges to commit page Changes/Builds tabs.
- Add "Builds" to commit Builds tab page title.
- Add and use Ci::Build#retryable? method.
- Add CI::Build#retried? method.
- Allow all failed commit builds to be retried.
- Proper authorization for cancel/retry all builds.
- Remove unused param.
- Use time_ago_with_tooltip where appropriate.
- Tweak builds index text
- Remove duplication between builds/build and commit_statuses/commit_status.
- Use POST rather than GET for canceling and retrying builds.
- Remove redundant URL helpers.
- Add build ID to build page.
- Link branch name on build page.
- Move commit/:sha/ci to commit/:sha/builds.

9722 fix alignment of event items and its children

Allow groups to appear in the search results if the group owner allows it

https://dev.gitlab.org/gitlab/gitlabhq/issues/2565

See merge request !1751

Otherwise text will flow under the "timeago" element.
The original value was 80px (see commit 8b0906c8), but a value of 174px
makes more sense. (see event-title `calc` function).

update example of regex for pytest-cov



See merge request !1745

fixed typo in installation manual