Move wikibase.experts from lib to repo

Change-Id: I10f39c1856053fbaf2bb71425e1f6154c651d7f0
Move some messages to client or repo

These are only used in client or repo, respectively.

Change-Id: I3dc58b8d7ae3d3520479513be81bab09471705a5
Merge "Move some messages to client or repo"