Added Sidetiq gem to implement periodically running jobs.

See: https://github.com/tobiassvn/sidetiq
WIP: make maintenance jobs run periodically using the Sidetiq gem.

CleanupInvitationsWorker runs daily at 2AM. When Sidekiq starts, if it detects that some runs of the worker have been missed (because the server was down, e.g.) they are scheduled immediately.
Sidetiq uses the rails log file.
Include Sidetiq tab in Sidekiq web ui.
Configure Rails in the development environment to eager-load code in the configured paths (which by default includes everything under /app) on startup.

In particular, this means that (among other things) Sidekiq workers are loaded on startup. This is necessary for the workers to appear in the Sidetiq tab of Sidekiq's web UI. The default Rails 4 configuration disables eager loading in development, which means that the mentioned Sidetiq tab does not display anything.

The price to pay is slower Rails startup in development. Oh well.

For more about this see:

https://github.com/tobiassvn/sidetiq/wiki/Known-Issues

and the lengthy discussion about the issue in:

https://github.com/tobiassvn/sidetiq/issues/12
Schedule CleanupSignupsWorker to run daily at 3AM with Sidetiq
Schedule DestroyOldJobStatesWorker to run daily at 4 AM with Sidetiq
Schedule FixSchedulesWorker to run daily at 5 AM with Sidetiq