Checking if the email already exists

It checks only against verified users. Any feedback is welcome..

P.S. Since there is no parent in the `User` model there might be issues with [eventual consistency](https://developers.google.com/appengine/docs/python/datastore/structuring_for_strong_consistency) thingy but in real life situations if there is ever going to be a conflict with that.. it could be solved with a support ticket :D
The shortest so far:
```python
return not user_dbs or self_db in user_dbs and not user_dbs[1:]
```
http://repl.it/XHC/8
http://codereview.stackexchange.com/a/60524/5138