Add token authentication

This changes `/reminders/email/send` to require an API token. The `/authenticate` endpoint is added to allow clients to receive a token.

This also removes the check that the Remind On date isn't in the past when firing off emails. This makes testing difficult and probably doesn't add much value anyway.

Todo
- [ ] Replace admin/admin with something real
- [ ] Determine if the rake task should just assume current day as the Remind On date
- [ ] Solve timeout issue when sending lots of emails