Added admin page for managing events

Admin users can now go to `/admin/` and see a list of all events, add an event, and edit the settings on any existing events.

It's obviously super rudimentary and there are lots of directions to go here in terms of event analytics, pre-populating sessions in the same UI, fixing layout issues, etc. But it's a first step and I want to get it on live so our workflow for creating events is smoother.

Also, shows the admin link on the main header if you're logged in as an admin so you can find it.