Support Facility logos to be served through the api

- It's a URLField
- Depends on trusting the admins not to insert dirty images to be sent to clients
- pls

Closes #37
3 jobs for 37-facility-logo in 1 minute and 33 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed whats-open-py2.7 #3776

00:01:23

passed whats-open-py3.5 #3777

00:01:31

passed whats-open-py3.6 #3778

00:01:33