Build a very basic subscription detail page in Phortune

Summary:
Ref T6881.

• Add a subscription detail page.

Minor cosmetics:

• Fix glyph, from "X" (old "X marks the spot" icon) to "diamond" (new gem icon).
• Name the initial account "Default Account" instead of "Personal Account", since this seems more general.

Test Plan:

And I got two full days to test that Jan 30/31 -> Feb 28 billing logic!

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11576