Subevents Feature

[yewchung]

Checklist

• [x] I have read the Contribution & Best practices Guide.
• [ ] My branch is up-to-date with the upstream master branch.
• [x] The tests pass locally with my changes.
• [x] I have added tests that prove my fix is effective or that my feature works(if appropriate).
• [x] I have added necessary documentation (if appropriate).

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.)

[cycomachead]

Replaced by #264 due to merge conflicts.