Introduce Wikibase\Repo\View

Change-Id: If984fd732cd29c16aef5def1535116e2b87cf745
Factor out generation of site links HTML into own class

This has the nice side effect of reducing the number of created SiteSQLStore
instances. Since every SiteSQLStore instance maintains its own cached version
of the sites list, which it first has to fetch from the DB or cache, this
reduces both rendering time and memory consumption.

Change-Id: Id2e1fba574b737c15fa2dc5ee4fb6152ca240c96
Merge "Introduce Wikibase\Repo\View"
Merge "Factor out generation of site links HTML into own class"