This method, #route_not_found, is executed as the final fallback for
unrecognized routes (as the name might imply.) We want to avoid
`#authenticate_user!` when calling `#route_not_found`;
`#authenticate_user!` can, depending on the request format, return a 401
instead of redirecting to a login page. This opens a subtle security
exploit where anonymous users will receive a 401 response when
attempting to access a private repo, while a recognized user will
receive a 404, exposing the existence of the private, hidden repo.

These are not required because MySQL is not
supported anymore

This change adds audit logs for user impersonation
when an admin starts/stops impersonating
another user.

As we noticed in
https://gitlab.com/gitlab-org/gitlab-ce/issues/56627#note_185828742,
clicking on the "Issues" tab often requests the same reference in
rendering Markdown.

This sets up all the basics for importing Phabricator tasks into
GitLab issues.

To import all tasks from a Phabricator instance into GitLab, we'll
import all of them into a new project that will have its repository
disabled.

The import is hooked into a regular ProjectImport setup, but similar
to the GitHub parallel importer takes care of all the imports itself.

In this iteration, we're importing each page of tasks in a separate
sidekiq job.

The first thing we do when requesting a new page of tasks is schedule
the next page to be imported. But to avoid deadlocks, we only allow a
single job per worker type to run at the same time.

For now we're only importing basic Issue information, this should be
extended to richer information.

- This can be accessed with Session.current and is restored after.
- Data can be stored under a key with NamespacedSessionStore

This change is a fairly straightforward refactor to extract the tracing
and correlation-id code from the gitlab rails codebase into the new
LabKit-Ruby project.

The corresponding import into LabKit-Ruby was in
https://gitlab.com/gitlab-org/labkit-ruby/merge_requests/1

The code itself remains very similar for now.

Extracting it allows us to reuse it in other projects, such as
Gitaly-Ruby. This will give us the advantages of correlation-ids and
distributed tracing in that project too.

- Externalize strings in controllers
- Update PO file