Require valid emails for registration

### Steps to reproduce
1. Register for Hypothesis using an incomplete email format (eg, missing a TLD at the end).
2. Registration succeeds, even tho email is invalid.

### Expected behaviour
Emails supplied at registration should meet basic format requirements.

### Actual behaviour
Registration succeeds with invalid emails.

### Additional details
This was discovered by seeing apparently successful user registrations with emails like the following (anonymized):

- ***@uio
- mail@***schroeder
- ***18295@hot
- ***0220@gmail
- ***vid519@dot
- ***uan@com
- ***.***@usherbrooke
- ***20007@aegean