Add timezone support to mw.lang:formatDate

This uses the timezone support added to core Language::sprintfDate in
Iea1f7842 to add support for various timezone-related formatting
characters, and to correct the output for 'c', 'r', and 'U' in local
mode.

This is related to bug 33454, which requests the same for
ParserFunctions' {{#timel:}}.

Bug: 33454
Change-Id: I1b92dc671051a6cc53e35ebd74d383448e16696c