Refresh OAuth tokens

:fork_and_knife: 

This PR adds a column to the `accounts` table to track OAuth refresh tokens provided by OC-ID. It also adds a worker which periodically queues jobs to refresh OAuth tokens before they go stale, and configures Sidekiq to send errors to Sentry when Sentry is configured. Finally, it adds a minuscule API health endpoint to expose the number of expired OAuth tokens.

We're using [Sidetiq](https://github.com/tobiassvn/sidetiq) to queue jobs for the aforementioned worker every 5 minutes. The Sidetiq scheduler grabs OC-ID accounts whose tokens are set to expire between 20 and 25 minutes from the worker's scheduled start time, and queues a job to refresh that account's token.

My biggest reservation with Sidetiq at the moment is that it doesn't appear to have a great story around changing the recurrence of a given worker. Based on [this comment](https://github.com/tobiassvn/sidetiq/issues/56#issuecomment-30224606) such a change would require directly deleting keys from Redis.