Geo: sync disabled wikis

Per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5772#note_74259279

Currently, if a wiki repository is disabled, we do not attempt to sync the wiki. This was introduced because wiki repositories often don't exist, so syncing them would fail.

Since then, we've added code that handles non-existent wiki repositories, and skipping disabled wikis is arguably a data-loss scenario in the failover case:

It's very arguably a bug that if you disable a wiki, do a failover, then re-enable it, you get an empty repo, whereas just disabling then re-enabling the wiki leads to the content being preserved.

We should modify the Geo synchronization logic so that disabled wikis are attempted.