Add a JSON healthcheck to email-alert-api

This healthcheck includes a few things:

- an `ok` message, which tells us that the app is responding
- a nested hash called `checks`, which contains:
  - `queue_size`: this is the length of the unprocessed Sidekiq queue,
     which can be graphed to indicate that our API is backed up
  - `queue_age`: a delta in seconds of the age of the oldest item in the
     Sidekiq queue, which can be used to indicate that there's a problem
     processing messages

Both of the metrics represented here are intended to be monitored using Nagios.
Merge pull request #48 from alphagov/add-healthcheck

Add a JSON healthcheck to email-alert-api