Fix member edit transaction validation so it works for both implicit and explicit account creation
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.
- Went through implicit first-time creation flow.
- Went through explicit second-time creation flow.
- Unit test now passes.
Reviewed By: chad
Maniphest Tasks: T12484, T12451
Differential Revision: https://secure.phabricator.com/D17692