Create a worker which refreshes OC-ID OAuth tokens

In order to refresh OC-ID OAuth tokens, we need to store the refresh
token provided by OC-ID. We also need to store the timestamp provided by
OC-ID which represents the time at which the given OAuth token will
expire. This commit adds a column to the accounts table to store the
former, and adds the code required to store both whenever a user
authenticates.
Schedule OC-ID OAuth token refreshes with Sidetiq

We're using the version on GitHub because the version published on
RubyGems has compatibility problems with Sidekiq 3.0.

We backfill Sidetiq jobs in an attempt to prevent gaps in token refresh
coverge due to deploys.

Ref: https://github.com/tobiassvn/sidetiq/issues/79
Send Sidekiq errors to Sentry
Index account OAuth expiration times
Expose the number of expired OAuth tokens through the API
Merge pull request #357 from opscode/refresh-tokens

Refresh OAuth tokens