Simplify fundraising maintenance mode

Check $wgContributionTrackingFundraiserMaintenance in GatewayFormChooser
and GatewayForm before rendering or redirecting to payment forms, add
test to make sure redirection happens when maint mode is true.

Change-Id: Iaf750e7100aec4ef7498bd62f49199a83541ca25
Localisation updates from https://translatewiki.net.

Change-Id: I0ab8eba8588ec515695da89f93b3a8f41590d5d8