Update 404 handling. Fixes #832.

- Uses webapp2's error handling for 404 errors.
- Changes previous redirects to /error/404 to just throw 404's in place.
Merge pull request #849 from gregmarra/404

Update 404 handling. Fixes #832.
Use webapp2.Route to handle routing. Makes URIs cleaner to read.

In a later change, we can update all the places we have hardcoded URIs to generate them programmatically from these named Routes.
Fix removing 404 handler
show event short name in admin list
Use webapp2's error handling instead of redirecting
404 on team pages if no events for given year
Merge pull request #851 from gregmarra/routes

Use webapp2.Route to handle routing. Makes URIs cleaner to read.