Email - Part 1

Work in progress.

Read #2312. 

> Send verification email on email add/change
> Add new page for verifying email.

Once this is done, existing users will be able to add a verified email to their Gratipay account.

- [ ] Add fields for hash and ctime
- [ ] Create a verification endpoint. Check for correct hash, and time < 1 week.
- [ ] Only update hash/ctime when the email address changes, and send an email to user whenever email is changed.