Scaffolding for Fund

Summary:
Ref T5835. This is all pretty boilerplate, and does not interact with Phortune at all yet.

You can create "Initiatives", which have a title and description, and support most of the expected infrastructure (policies, transactions, mentions, edges, appsearch, remakrup, etc).

Only notable decisions:

  - Initiatives have an explicit owner. I think it's good to have a single clearly-responsible user behind an initiative.
  - I think that's it?

Test Plan:
  - Created an initiative.
  - Edited an initiative.
  - Changed application policy defaults.
  - Searched for initiatives.
  - Subscribed to an initiative.
  - Opened/closed an initiative.
  - Used `I123` and `{I123}` in remarkup.
  - Destroyed an initiative.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5835

Differential Revision: https://secure.phabricator.com/D10481