Create {{#avatar:Username}} parser function

Allowing size as a second parameter, giving a size code (s/m/ml/l), or a
px value, with the parser function selecting the appropriate size avatar
to load for the given px value

Change the way getAvatarURL() constructs the <img>, using HTML::element,
and allowing other parameters to be passed in.

I'll notice all these style errors on my own someday!

Bug: 34008
Change-Id: I54257a4a142bff1ae2b5026d57f8cb29fff24ad3
Merge "Create {{#avatar:Username}} parser function"