Add TOS and "I am over 13 years old" fields to registration form

The TOS checkbox is a required field.

The "I am over 13 years old" checkbox value is recorded in the user
table.

Fixes #166
Add registration UI tests

This adds coverage for TOS and password validation and makes sure the
user is_minor flag is correct based on input from the registration form.
Merge pull request #178 from kdeloach/topic/registration-fields

Add TOS and "I am over 13 years old" fields to registration form