Reduce code duplication in Phortune account controllers

Summary:
Ref T12451. This is a GREAT comment (A++) but we only need one copy of it.

This uses a pattern similar to Projects, which is a little weird but works well enough.

Test Plan:

• Viewed all four tabs of an account.
• Viewed a page with a bad account ID which 404'd properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12451

Differential Revision: https://secure.phabricator.com/D17694