Add RL template module with HTML markup language

Preparation work for templating in core.
RL should allow us to ship HTML / template markup from server to client.
Use in Special:Upload and mediawiki.feedback as a proof of concept.
Separation of concerns etc...

See Also:
Ia63d6b6868f23a773e4a41daa0036d4bf2cd6724

Change-Id: I6ff38c12897e3164969a1090449e626001926c3b
Merge "Add RL template module with HTML markup language"