Email - Part 1

Work in progress.

Read #2312. 

> Send verification email on email add/change
> Add new page for verifying email.

Once this is merged, existing users will be able to add a verified email to their Gratipay account.

- [x] Add emails table.
- [x] Create a verification endpoint. Check for correct hash, and time <  24 hours(?).
- [x] Add UI to show verification status on account page
- [x] Add resend verification email button, and corresponding notifications
- [x] Create mandrill test key for Gratipay (need help here)
- [ ] comprehend security concerns wrt injection into email templates
- [x] send notification to old email address
- [x] standardize on nomenclature: "confirm" vs. "verify"
- [x] i18n for emails (skip for now)
- [ ] i18n for callback (skip for now)
- [x] review and improve success/failure messaging on callback
- [x] add uniqueness constraint to email address

After this is merged, we'll have to: 

- [ ] Utilize the elsewhere email addresses and those in the `email` column to send initial verification emails.
- [ ] Delete the `email` column once the above step is done. 

Should we really use elsewhere? Those emails were never given to us, at most we could send an email inviting those users to add that email to their Gratipay account, but we shouldn't just add them without asking. If we want to take emails from elsewhere accounts we should do that during sign-up. Until we do that during sign-up it doesn't make sense to do that for existing elsewhere's since we'll just start accumulating more elsewhere emails right away again (unless this PR also silently grabs elsewhere emails during sign-up, which I think we shouldn't do; we should ask).