add Echo support

• Sprinkle debug statements throughout to help with that.
• Add shim to work w/o WatchedItemStore from MediaWikiServices.
• Make third arg on CategoryAfterPageRemovalHook optional.
• Add title-message key that older Echo requires and silently dies without.

Bug: T168072
Change-Id: Id0df74ccef1f9c6ea51c22396977f5424cbe19ae