Implement session expiry checking for proofread actions

Breaking changes:
* Removed mw.translate.getProofreadToken
* Removed mw.translate.proofread

The above functions are no longer needed since MediaWiki 1.22
supports postWithToken. There is a bug in postWithToken which
I worked around. See Ibe48956a for a suggested fix in core.

Tweaked the CSS a little bit to allow tux-warning to show up
and tweaked the image height to avoid overlow below the icon.

Small refactoring in the code to remove proofread = this and
to avoid requering some elements again.

Bug: 69314
Change-Id: Icd20eaa58451fc52d550e650afb3016806f664a6
Merge "Implement session expiry checking for proofread actions"