Feature summary (what you would like to be able to do and where):
Would be nice to expose MediaWiki:watchlist-expiry-options via Community configuration instead of via the very basic MediaWiki key.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
As a user you need to know about the existence of MediaWiki:watchlist-expiry-options and the fact that these values are customizable to begin with.

Benefits (why should this be implemented?):
Easier usage and better discoverability of these options