R1:505b1d8379a5

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.