Short description of what this resolves/which issues does this fix?:
Implements the Subevent feature (also there's some other changes inherited from our base repo)
Changes proposed in this pull request:
Adds two new columns to the Events model (superevent and parent_id)
Filters out all subevents from the main schedule page so they aren't displayed twice
Displays all subevents inside of their associated parent's event box
Displays all subevents inside of a parent's "show" page
Creates options on an event's "edit" page allowing a user to toggle superevent and/or choose a parent event id.
(Note that it's technically possible for a subevent to be a superevent as well, allowing for chaining subevents. Since an event's subevent display is skipped in compact view this won't currently display on the schedule, but by detabbing that section you can make that a feature if you want.)
This is useful for lightning talks
Replaces #247