Fix inconsistencies in `ProjectRegistry` queries

From https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5104#note_64850154:

The meaning of some ProjectRegistry fields overlap, so there isn't a SSOT, which has opened us up to inconsistencies in the queries.

• synced_wikis are last_wiki_synced_at is not null and last_wiki_successful_sync_at is not null and resync_wiki = 'f'
• But RepositoryShardSyncWorker only schedules resync_wiki = 't' and doesn't look at last_wiki_synced_at or last_wiki_successful_sync_at

Here's another inconsistency: failed_wikis are based on wiki_retry_count.

To summarize:

• synced + unsynced + failed should always equal all
• But currently this is not guaranteed to be true.

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• Fix the queries
• Simplify the ProjectRegistry fields? (This sounds daunting)