Dyanmic Error Pages, Part 1

Shouldn't functionally change anything we have at the moment, so it
should be safe to merge.
The first step toward having dynamic error pages. This also fixes the
back link so it will go to the most recently used payment form, instead
of some crazy place on donate.
What's missing: The switch that will enable us to use this error page,
instead of the static one.
Also, the appeal param isn't sticky, so if you're relying on css in
on-wiki appeals to alter forms, STOP IT.

Change-Id: I6b1baffd664ae814740bf57774c9f62859a3eefc
Localisation updates from http://translatewiki.net.

Change-Id: I8ddc0608c370792bbcdd15aeefca254723b83192
Allow a GatewayFormChooser Selection Bias

This will allow us to remove a form from random selection but not
completely take it out of form choosing in case we need to test or
if it's the only option.

Change-Id: Ia0038c9f5aae3542e2f28c2da033c2b51b20b2b6