Proposal: Forms action type

A more generic action type than petitions, to allow for things like signup forms on a website to more complex actions like surveys. Works with questions and question answers. Takes the same format as our other action types.

New fields being proposed:

call_to_action: The call to action of a form, describes the action you're asking users to take.

signups: The granular action created when an activist signs up on a form -- similar to signatures for the petition action type and attendances for the event action type.