Implemented AMS Rebuilds: Rebuild AMS Article Cover Image Cache

[Bob]

I've added a new AMS Rebuild - Rebuild AMS Article Cover Image Cache

This rebuild function was part of the Rebuild AMS Articles rebuild, however, that rebuild was getting a bit to heavy, so I split this process into its own rebuild. This rebuild only handles rebuilding the cover image cache for AMS articles.

I've located this rebuild directly below the Rebuild AMS Thumbnails as when ever you run the Rebuild AMS Thumbnails, you need to run the Rebuild AMS Article Cover Image Cache.