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.

Closes #34052

This is merely to make CE and EE more compatible.
See the EE merge request at:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2112

This adds an ID-less table containing one row per file, per merge request
diff. It has a column for each attribute on Gitlab::Git::Diff that is serialised
currently, with the advantage that we can easily query the attributes of this
new table.

It does not migrate existing data, so we have fallback code when the legacy
st_diffs column is present instead. For a merge request diff to be valid, it
should have at most one of:

* Rows in this new table, with the correct merge_request_diff_id.
* A non-NULL st_diffs column.

It may have neither, if the diff is empty.

Create a post-deployment migration to update all existing notification settings
with at least one custom level enabled to the new format. Also handle the same
conversion when updating settings, to catch any stragglers.

Add columns for each custom notification level, defaulting to null. Read from
those columns if non-null, otherwise fall back to the serialized column. Writing
will write to the new column if `events` isn't manually set.

Ran:
 - git format-patch v9.2.2..v9.2.5 --stdout > patchfile.patch
 - git checkout -b 9-2-5-security-patch origin/v9.2.2
 - git apply patchfile.patch
 - git commit
 - [Got the sha ref for the commit]
 - git checkout -b upstream-9-2-security master
 - git cherry-pick <SHA of the patchfile commit>
 - [Resolved conflicts]
 - git cherry-pick --continue

Ran:
 - git format-patch v9.2.2..v9.2.5 --stdout > patchfile.patch
 - git checkout -b 9-2-5-security-patch origin/v9.2.2
 - git apply patchfile.patch
 - git commit
 - [Got the sha ref for the commit]
 - git checkout -b upstream-9-2-security master
 - git cherry-pick <SHA of the patchfile commit>
 - [Resolved conflicts]
 - git cherry-pick --continue

* implemented logging of project and system web hooks
* implemented UI for user area (project hooks)
* implemented UI for admin area (system hooks)
* implemented retry of logged webhook
* NOT imeplemented log remover

We introduce ci_builds.environment_url and expand it when passing to
runners. In the future the deployment could also retrieve the URL from
the job instead of from the environment, which might have a different
external URL because of another deployment happened.