Add some more reserved names.

I just browsed around and noted down the non-code URLs where the sidebar popped up. There might be more I haven't noticed...

`about` is used for the about pages (e.g. github.com/about/team)
`orgs` is used when you view the dashboard for an organization you're in.
`styleguide` is e.g. https://github.com/styleguide/css/7.0
`showcases` and `trending` are used in the Explore feature
`stars` is used to see a user's stars (e.g. github.com/stars/:username)
`dashboard` is used for the personal dashboard
`notifications` is e.g. https://github.com/notifications/participating

There are some more (e.g. `watching`) but they don't seem to have subpaths so they don't match the regex anyway.

I also noticed that `followers` and `following` are reserved repo names (not sure why; I'd have expected e.g. github.com/followers/:username instead of github.com/:username/followers).