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. 

Because for now we only use it for emails, and because it seems to me like a good idea to be able to choose the language of emails independently from the rest.