R1:09fb5667cc49

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…