Create reminders

* Move back end url to config to facilitate changing environments.
* Create reminders. Currently they are created individually, with a message saying so even before they are all asynchronously created. I plan to create a bulk action so the "reminders created" message won't be optimistic.

@codeforamerica/denver this is the first hack at this. It needs some iteration. Start thinking about:
:calling:  The email/sms wording.
:ear:  The location and styling of error message.
:e-mail:  The feedback to the user when reminders are created. Obvs we will use the all mighty modal, but do we want to display a list of all the dates they'll be reminded?
:o: Other things