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: Ib09e8eee78b5b360d16deac8604fdf99c544c994
Merge "Simplify fundraising maintenance mode"