Backport changes from introducing GithubService interation in EE

See merge request gitlab-org/gitlab-ce!17607

Don't delete todos or unassign issues and MRs when a user leaves a project

Closes #43899

See merge request gitlab-org/gitlab-ce!17615

Port changes from EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4885 (Maintainer push to fork)

See merge request gitlab-org/gitlab-ce!17629

Removes confusing/unnecessary checkboxes when trying to configure an
integration. If there is only one supported event we don't need to
allow these to be individually disabled since the integration can be
disabled instead.

E.g. Project Integrations for GitHub, Bugzilla, Asana, Pipeline emails and Gemnasium

Allows integrations to override which triggers are configurable

backport of custom metrics for the common metrics section of CE

See merge request gitlab-org/gitlab-ce!17586

Update project activity when pushing to a wiki repository

See merge request gitlab-org/gitlab-ce!17621

Create CI/CD-only projects from GitHub -- CE backport

See merge request gitlab-org/gitlab-ce!17432

Fix accessibility typo

See merge request gitlab-org/gitlab-ce!17612

Backport relevant changes from EE https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4827 to CE

See merge request gitlab-org/gitlab-ce!17608

Move SidebarAssignees vue component

See merge request gitlab-org/gitlab-ce!17398

Resolve "Add a mention and link to doc for security implications in the Kubernetes cluster page"

Closes #43780 and #43781

See merge request gitlab-org/gitlab-ce!17486

# Conflicts:
#   locale/gitlab.pot

Add new frontend guide structure

See merge request gitlab-org/gitlab-ce!17611

Allow maintainers to push forks of a project for branches that have open MRs

Closes #22292

See merge request gitlab-org/gitlab-ce!17395

Update GitLab Pages to v0.7.0

See merge request gitlab-org/gitlab-ce!17603

Resolve "Make margin around dropdown dividers 4px"

Closes #39444

See merge request gitlab-org/gitlab-ce!17517

Reduce number of FindCommit calls from RefsController#logs_tree

Closes gitaly#1057

See merge request gitlab-org/gitlab-ce!17566

Fixes attempts to update a service integration which had `can_test?`
set to true but validations were causing the "Test and save changes"
button to return "Something went wrong on our end."

Removes references to index action left from 0af99433

Adds detailed_status to pipeline hook data
Adds detailed_description option for Services
Integration edit page renders 404 if a service is disabled

Fix a JS bug in the websocket terminal

See merge request gitlab-org/gitlab-ce!17470

Change to Pacific Time Zone

[ci skip]

See merge request gitlab-org/gitlab-ce!17600

Upgrade grpc gem to 1.10.0

See merge request gitlab-org/gitlab-ce!17575

 Conflicts:
   GITLAB_PAGES_VERSION

Upgrade Workhorse to 4.0.0

See merge request gitlab-org/gitlab-ce!17606

Resolve: Modal and banner designs for project label and project milestone promotion

Closes #41895

See merge request gitlab-org/gitlab-ce!17197

Minor fixes in API doc

See merge request gitlab-org/gitlab-ce!17602

Use Project#full_name instead of name_with_namespace

See merge request gitlab-org/gitlab-ce!17601

We only allow users that can merge the merge request to push to the
fork.

When creating a cross project merge request without changes. The MR
widget shows the option to create a new file. It would however link to
the wrong project.

Using the relation `Project#source_of_merge_requests` in
`Ci::CreatePipelineService`

That way we take advantage of all the `ActiveRecord::Relation` magic