server: verify semaphore proof upon creation of first post or comment in anonymous group:

[sripwoud]

will need to:

1. create nullifiers, roots... tables see yeap tables, bandada boilerplate
2. on proof verification (see yeap)
   1. Check that nullifier wasn't already used
   2. Check that the group root has not changed or that is has not expired
   3. Verify proof: use https://github.com/adria0/semaphore-rs-backend
   4. Insert nullifier in nullifier table