Reliable tests for all of ExperimentAgent
New sign-up process onto trial account; more work on changing plan
Nightly update implementation on backend and frontend

Frontend:
- Coupon code and reset usage can be controlled from the frontend
- Simplified display of dates (dates only, not times)
- 'go' parameter in signup form, which controls the next URL visited

Client:
- Nightly update methods added

Backend
- Better quota checks and warning emails
  - Free-plan users are cut off when they go over 110% quota
  - All other users can continue to user Myna indefinitely
  - When any user goes over quota, we send a warning email to hello@mynaweb.com
  - When RecurlyInterface.syncSubscription fails, we send a warning email to hello@mynaweb.com
Merge commit 'da9919fdad6546173371f16d5e221b37e83d479f'