Use plural for 'wikibase-comment-multi' and numParams()

Even though English only has a single plural form, that's not true for
many other languages. Plurals should be declared explicitely where
possible. Use wfMessage()->numParams() for parameters that are numbers, so
number conversion is used where applicable.

Change-Id: I4070ddd85262b8a3f6de10719f250ad2cb8ad329
Merge "Use plural for 'wikibase-comment-multi' and numParams()"