This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

This removes the need for relying on Rails' "dependent" option for data
removal, which is _incredibly_ slow (even when using :delete_all) when
deleting large amounts of data. This also ensures data consistency is
enforced on DB level and not on application level (something Rails is
really bad at).

This commit also includes various migrations to add foreign keys to
tables that eventually point to "projects" to ensure no rows get
orphaned upon removing a project.

This reverts merge request !12557

In Geo, Project#ensure_repository is used to make sure that a Git repo
is available to be fetched on a secondary. If a project were a fork,
this directory would never be created.

Closes gitlab-org/gitlab-ee#2800

This reverts commit b07c0003, reversing
changes made to 2b97d76d.

Remove unnecessary prometheus_service method, that is now handled by new automagic within Project model

This optimises how GroupProjectsFinder builds it collection, producing
simpler and faster queries in the process. It also cleans up the code a
bit to make it easier to understand.

This method used to use a UNION, which would lead to it performing the
same query twice; producing less than ideal performance. Further, in
certain cases ActiveRecord could get confused and mess up the variable
bindings, though it's not clear how/why exactly this happens.

Fortunately we can work around all of this by building some of the WHERE
conditions manually, allowing us to use a simple OR statement to get all
the data we want without any of the above problems.

In EE we don't want to remove import data for mirrors. By moving this
logic to a separate method we can simply re-define the method in EE
using "prepend" and call "super" whenever necessary.

This ensures the order of these methods is identical in CE and EE.

To avoid the use of slow queries, remove some deprecated methods and encourage
the use of ProjectFinder to find projects.

Add a scope to search for the projects that are starred by a certain user.