Make the maximum capacity of Geo backfill operations configurable

What does this MR do?

Moves repository and file maximum backfill capacity into the database and out of hardcoded constants

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

The default values come from the existing constants, so this isn't a behavioural change.

Administrators can now prioritize one secondary over another using these tweaks.

Why was this MR needed?

Administrators may wish to tweak the amount of load a Geo secondary's backfill operations place on the primary

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added no API support
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • 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 #3532 (closed)