Make Phortune account members editable and modernize the edge constant

Summary:
Ref T2787.

• Account members can add and remove other members (major use case is corporate accounts).
• Use a modern edge constant setup.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10678