We cache whether a project has an External Wiki integration enabled in
the `has_external_wiki` column on `projects`. It will be `TRUE` if the
project has the "External Wiki" integration (service) enabled.

This was previously maintained with application code.

This cache is easy to fall out of consistency when we fail to maintain
it during bulk operations (including PostgreSQL cascading deletes
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48163/) as
mentioned in:

- https://gitlab.com/gitlab-org/gitlab/-/issues/273574#note_457006686
- https://gitlab.com/gitlab-org/gitlab/-/issues/289798

As Ecosystem is increasingly changing integration data using bulk
operations this has been changed to be maintained through a PostgreSQL
trigger.

https://gitlab.com/gitlab-org/gitlab/-/issues/290715
https://gitlab.com/gitlab-org/gitlab/-/issues/289798