Bring from EE: Share Project with Group

- [x] Models and migrations
- [x] Logic, UI
- [x] Tests
- [x] Documentation
- [x] Share with group lock
- [x] Api feature
- [x] Api docs
- [x] Api tests
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

For #12831 

cc @DouweM @rspeicher @vsizov

See merge request !3186

Prepare docs for 8.6 RC1

[ci skip]

See merge request !3173

Don't remove `ProjectSnippet#expires_at` from API

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3103

This partially reverts commit 836d5930.

See merge request !3144

[ci skip]

Fixed tests for MR & issue filters

See merge request !3203

Remove sleeps from network graph feature spec

Also removed a double negative in the step description

See merge request !3199

Use Capybara find methods and remove sleeps in feature specs in "All Issues" filter

Fixes sporadic build failures (e.g. https://gitlab.com/gitlab-org/gitlab-ce/builds/856233)

See merge request !3198

Implements upcoming filter in milstones

![Imgur](http://i.imgur.com/Oc2LZrF.png)

@DouweM @JobV I think this is what you wanted :) Lemme know if it there is something that need changing
Had alot of fun doing it!

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14102

See merge request !3148

Redirect host check

Fixes https://dev.gitlab.org/gitlab/gitlabhq/issues/2649

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/13956

See merge request !1945

Fixes
https://dev.gitlab.org/gitlab/gitlabhq/issues/2649
https://gitlab.com/gitlab-org/gitlab-ce/issues/13956

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

Reduce example documentation.

We should not duplicate indexes between the top level and lower level.

See merge request !3162

Revert "Revert "Merge branch 'support-go-subpackages' into 'master'""

This reverts commit 5a586f36

See merge request !3194

This reverts commit 5a586f36

Remove unnecessary go-get test (superseded by middleware)



See merge request !3193

This reverts merge request !3191

Support Golang subpackage fetching

Closes #13805

See merge request !3191

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

Removed User#project_relations

This is to make things more compatible with EE so merging is easier.

cc @dzaporozhets 

See merge request !3192

GitLab EE adds an extra relation that selects a "project_id" column
instead of an "id" column, making it very hard for this method to be
re-used in EE. Since using User#authorized_groups in
ProjectsFinder#all_groups apparently has no performance impact we can
just use it and keep everything compatible with EE.

Filter import_url params because they may contain auth information. Fixes #14199

cc @dzaporozhets @DouweM tagging you since this is security related.

See merge request !3174

Fix importing PR's from GitHub when the source repo was removed

Closes #13847 
Closes gitlab-com/support-forum#584

See merge request !3172