Make FormatMetadata accept RequestContext, instead of hard coding $wgLang.

Make FormatMetadata accept RequestContext, instead of hard coding $wgLang.

The point of this patch is to allow someone to use this class
to get the metadata formatted in the language of their choice
(by passing in an appropriate context object), instead of
forcing $wgLang. This is important for future evil plans I
have.

To do this, some of the previous static methods were changed
to non-static. I checked and fixed all users of such methods
(including extensions). Some of the implied private methods
were also explicitly marked as such.

Change-Id: I673d99fa36280d1baf14c150f4aaff039676830a