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

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

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

The query becomes a lot simpler if we can check the branch name as
well instead of having to load all branch names.