Support for Recurring Events

## Planning issue for recurring events
Currently events are set for a specific date, and do not recur. It's been suggested that people might like to put repeating events in the system similar to the functionality that we have in on WebSiteOne.
[Ticket link on pivotal tracker](https://www.pivotaltracker.com/n/projects/742821/stories/151086152)

I am breaking this to be able to measure progress and submit bits by bits until its done. Anyone interested is welcome to help. :smiley:

### SubTasks

* [ ] Add gem recurrring-select/ ice cube and add recurrence rules on the model.
* [ ] Display recurring events on the browser + the non-recurring events.
* [ ] Add another event-like model(with a horizon)---events instances table.
* [ ] Rake task to update the "event-like" model every time an event is created/changed. Basically clears them all and recreates them.
* [ ] Tests for the recurring events functionality
* [ ] Optimize the events instances query (maybe consider caching)