Implement custom fields in Projects

Summary:
Ref T4379. Ref T3794. Fixes T4010. This brings CustomFields to projects.

My primary goal is to get rid of the special casing around project profiles and profile editing, so all edits are ApplicationTransactions. Particularly, I want to make the "blurb/description" field a custom field which goes through builtin infrastructure.

A distant secondary goal is that this is a feature which users like/want because users like/want features.

Test Plan: Added a custom field and examined it in the edit, view, and search interfaces.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3794, T4010, T4379

Differential Revision: https://secure.phabricator.com/D8180