Geo: Don't sync disabled project wikis

What does this MR do?

Stops Geo secondaries from attempting to sync disabled wikis

Are there points in the code the reviewer needs to double check?

An interesting question is what happens if a wiki is disabled, then enabled later.

In this MR, as long as the wiki was never previously synced, then the update will be picked up the next time the project's repository changes. However, if the wiki was previously synced then it won't be picked up.

I expect we'll need to add another event to the log cursor, indicating that the wiki was enabled, to be able to cover this eventuality. I'd prefer this to happen in a separate MR.

Why was this MR needed?

Currently, we repeatedly attempt to sync wikis that don't exist. This places an unwelcome load on the primary and causes the secondary to believe that large numbers of syncs are failing, when in fact they should not be happening at all.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3569 (closed)