Various improvements to repository checks

See merge request gitlab-org/gitlab-ce!18484

Live trace: Use efficient destroy all (for `dependent: :destory` problem)

See merge request gitlab-org/gitlab-ce!18575

New CI Job live-trace architecture (v2)

Closes #44935

See merge request gitlab-org/gitlab-ce!18169

Only run session related Warden hooks for user scope

Closes gitlab-ee#5857

See merge request gitlab-org/gitlab-ce!18771

Resolve "Vuex actions - How to achieve a better state management"

Closes #44427

See merge request gitlab-org/gitlab-ce!17929

Resolve "Improve Web IDE left panel and modes"

Closes #44846

See merge request gitlab-org/gitlab-ce!18581

Upgrade underscore.js

See merge request gitlab-org/gitlab-ce!18578

Enable prometheus metrics by default

Closes #43660

See merge request gitlab-org/gitlab-ce!18723

Add signature verification badge to compare view

See merge request gitlab-org/gitlab-ce!18245

Resolve "CI retry/cancel job or pipeline redirect the user and can't be open in a new tab"

Closes #33697

See merge request gitlab-org/gitlab-ce!18451

Backport CE changes from "autocomplete usernames in Epic comments/description"

See merge request gitlab-org/gitlab-ce!18605

Remove top margin on the terms page with performance bar

Closes #46064

See merge request gitlab-org/gitlab-ce!18752

For easier overriding methods.

No idea where it used to be, but I can no longer find it.

A repo does not need pushes before it will be repo-checked. There are
other sources where commits can come from (e.g. import or webIDE).

In the test I made the method of breaking a repo more robust. And
added a few test examples.

See also gitlab-org/gitlab-ce#45046.

In case the repository checks are disabled, do not send the
mails. Otherwise it might send the same failures over and over again.

Update instalation from source guide

See merge request gitlab-org/gitlab-ce!18691