Allow users to back initiatives in Fund

Summary:
Ref T5835. This is still completely made up (no payment integration), but you can "back" an initiative, type a number in the box, and generate a database row. You can then seach for backers and things you've backed and such.

Notable changes:

  - Renamed "FundBacking" to "FundBacker". The former name was sort of because you can back things multiple times, but stuff like `$backings` was just too weird.
  - I think that's it?

Test Plan:
  - Backed an initiative.
  - Viewed that I became a backer.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5835

Differential Revision: https://secure.phabricator.com/D10486