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
there are basically these outcomes..:

- `count == 0`: username is available
- `count == 2`: username is not available, because most likely is a conflict but that's another story
- `count == 1`: here there are two things that could happen.. 
  - if you found yourself then it's OK and the username is available 
  - if it's not you then username is not available

So all of the above can be translated to 

```python
username_available = not (count == 2 or count == 1 and self_db.key != user_dbs[0].key)
```