Allow trailing slash in URLs

This fixes issue #985 by introducing redirects that allow for a trailing slash in the URL. I was mostly able to use `RedirectRoute()`, but I ran into an issue with routes such as the ones for events.

The URL `/events/` is picked up by `/events/<year:[0-9]+>`, where we actually want it to be picked up by `/events`. It resulted in a "Missing argument year to build URI" error. So I left events, insights, and teams using `webapp2.Route()`. Maybe someone better with regex can make the routes for those three a little cleaner.