Fix member edit transaction validation so it works for both implicit and explicit account creation

Summary:
Ref T12451. Ref T12484. This should deal with all the + / - / = cases correctly, I think.

Also makes sure that members are real users, not commits or tokens or whatever. And expands the creation test case to make some other basic sanity checks.

Test Plan:

• Went through implicit first-time creation flow.
• Went through explicit second-time creation flow.
• Unit test now passes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12484, T12451

Differential Revision: https://secure.phabricator.com/D17692