Don't show removed or deleted links in the discussion bar.

Fixes #23.
Add simple way to prevent link from showing in discussions.
Change adding contributors to inviting contributors.

This mirrors the similar system for subreddit moderators.
Split out a base page that isn't event-context dependent.

This pulls some of the basic liveupdate stuff out to LiveUpdatePage and
moves the event-specific page stuff into LiveUpdateEventPage.  The
latter name was already used but that was a terrible name so the old
LiveUpdateEventPage becomes LiveUpdateEventApp which is closer to the
truth anyway.

The goal here is to have liveupdate CSS and JS available in non-event
contexts for e.g. listings of events.
Add system for reporting rule-breaking streams.
Add an event creation form for admins.
Fix regression of first-update not showing.

This regressed in the backbonification. :(