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.

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2112

This reverts merge request !12557

Signed-off-by: NRémy Coutable <remy@rymai.me>

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

Group#destroy would actually hard-delete all associated projects even
though the acts_as_paranoia gem is used, preventing Projects::DestroyService
from doing any work.

We first noticed this while trying to log all projects deletion to the Geo
log.

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

These attributes are stored in binary in the database, but exposed as
strings. This allows one to query/create data using plain SHA1 hashes as
Strings, while storing them more efficiently as binary.