Resolve "Follow-up from "Use scoped routes for labels and milestones""

Closes #63177

See merge request gitlab-org/gitlab-ce!29983

Pipeline must be present if "Pipeline must succeed" is set

See merge request gitlab-org/gitlab-ce!29926

This is largely a straight-forward copy of the Karma spec, with the
usual Karma->Jest necessary changes.

This also fixes a bug in the implementation that was causing the tests
to fail, when run on Jest/node, whereby actions that return rejected
promises would always fail the given test. This was due to those
rejections not being caught in the same call stack in which they were
created.

The setup was wrong, and due to Gitaly being a bit more strict it had to
match up. The test is now accurate and passing again.

Prevent EE backport migrations from running if CE is not migrated

Closes #63612

See merge request gitlab-org/gitlab-ce!30002

If a user upgraded to any GitLab 11.x EE version but switched
back to CE, it's possible the state of the EE tables are not
in the right state for the EE backport migration to work properly.
In particular, there were three tables that had trouble:

* epics
* geo_event_log
* vulnerability_feedback

The EE backport migration would fail while trying to add foreign key
constraints because a key didn't exist in the table. This happens
because any EE migration that add or removed columns between v11.0.0 and
v11.11.3 are not guaranteed to be applied in an CE installation. The EE
backport schema does not individually backport these migrations.

We now check if certain columns are present to determine whether
the backport migration is in the proper state. CE users are required
to upgrade to v11.11.3 EE if they ever installed EE previously before
they can go back to v12.x CE.

Tested via:

```
git checkout -f v11.0.0-ee
bundle exec rake db:reset
git checkout .; git checkout -f v11.11.3
bundle exec rake db:migrate
git checkout .; git checkout -f v12.0.0
bundle exec rake db:migrate
<failure happens>
```

When removing the last file from a folder in WebIDE, do not remove the
folder from the view. We keep those to give users possibility to
re-upload files to the same folders.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60860

The `:` key will no longer exit out of the autocomplete dialog,
allowing auto-completion of labels with colons in them.

When a user sets only_allow_merge_if_pipeline_succeeds, also named
as "Pipeline must succeed" project setting, we require the pipeline
to be present.

This solves race condition issues especially with external CI
providers when a build is triggered externally but no pipelines are
created in GitLab yet.

Document that a head pipeline is expected when using "Pipeline
must succeed" setting. Also explain limitations with the use of
only/except where there may not be any jobs created and the merge
request will not be allowed to be merged.

Also add ::Gitlab::LetsEncrypt.enabled? shortcut
and simplify it a lot