Add notice about GitLab Runner to requirements docs

This is related to #14589, and problems that may stem from running
GitLab Runner on same machine user installed GitLab web app on.

See merge request !3518

Add configurable shared runners text

cc @axil @rspeicher @grzesiek 

See merge request !3750

* 'master' of dev.gitlab.org:gitlab/gitlabhq:
  Add Changelog entry for group link permissions fix
  Use guard clause to check ability to share project
  Refactor method that shares project with a group
  Check permissions when sharing project with group

Check permissions when sharing project with group

## Summary

Unprivileged user was able to share project with group he didn't have access to, and therefore gain partial access to that group, which opened possibilities for further actions like listing private projects in that group.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/15330

## Fix

This change introduces additional check for group read access.

## Further work

We can think about preventing such problems in the future (this is quite common problem) by moving permissions checks to another layer of abstraction (TBD).

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15330

See merge request !1949

Make before_script and after_script overridable

This is makes it possible to overwrite the before_script and after_script at job level.

This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3771


See merge request !3772

Implement after_script which allows to do cleanups as part of the build process

This implements `after_script` in global context.

The `after_script` will be executed always after the job, even if the job were canceled.

This requires changes on Runner side that will be implemented in 1.2.

cc @tmaczukin @grzesiek 


See merge request !3771

Allow enable/disable push events for system hooks in UI

Improvement for !3744 

Fixes #15284 

See merge request !3812

Closes #15330

Fix prepare build execution in docker environment

In newest versions of docker the `/.dockerinit` file doesn't exists any more (https://github.com/docker/docker/pull/19490). And oo our builds are failing, because the `./scripts/prepare_build.sh` script isn't installing required dependencies.

This MR fixes the problem with backward compatibility for oldest docker environments. 

/cc @grzegorz @douwe 

See merge request !1950

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

Added System Hooks for push and tag_push

Implements `push` and `tag_push` events for System Hooks.

They are based on Webhooks Events but without any deprecated item. We don't send commits too.
We are implementing this, to be used by Geo repository syncing (gitlab-org/gitlab-ee#76).

These hook events will be sent only when the respective configuration flags are set to true.
UI changes to admin screen will be made in another MR.

See merge request !3744

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

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

Code is based on Project Webhooks, removing deprecations and without
commits listing.

Improve 'auto fsck' feature

- skip wiki repositories when they are disabled (avoids false alarms)
- more context in admin alert emails

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15332

See merge request !3781

Import milestones from GitHub

Closes #13421

See merge request !3797

Import labels from GitHub

Closes #15292

See merge request !3776

Use GitHub Issue/PR number as iid to keep references

Closes #15294 

See merge request !3759

Fixed issue with multi select not working with arrow keys



See merge request !3792

Add null check to formData.append

Fixes https://gitlab.com/gitlab-com/support-forum/issues/645

See merge request !3799

Change the link color of the target branch in a merge rquest

closes #15296

See merge request !3786

Use raw SQL commands for 20140502125220 MigrateRepoSize

Partial fix for #15210

See merge request !3789