Allow specifying different license for "data" namespaces

This patch puts handling of copyright message into a new class,
with tests.

New settings are added to set license text and url for entity content,
defaulting to $wgRightsText and $wgRightsUrl, but can be set to
something different. (e.g. in cases where wikitext and data
have different licenses)

This also moves wikibase-save and wikibase-shortcopyrightwarning
messages from WikibaseRepo to WikibaseLib since those are used there.

Bug: 59206
Change-Id: Ib56092456278822f82afb69612c1b0626212e5c5