Move gateway form logic from execute to handleRequest

Move execute method from GatewayForm subclasses into GatewayForm
so it can run global checks before executing gateway-specific logic.
As awight points out, this is a baby step towards separating the mediawiki
stuff from the gateway-specific stuff.

Remove unused page parameters from handleRequest, make GatewayForm
an abstract class and handleRequest and abstract method.

Change-Id: I934619425cbec2eb77e322e2b64ddd76499ef6fb
Merge "Move gateway form logic from execute to handleRequest"