finally good /c/ and /user/ matching

finally good /c/ and /user/ matching

don't look up both types of channels and return just one, but do the
correct lookup in the first place. no point in doing the fallback lookup
now that i'm fairly certain the lookup does the right thing.

note: using @app.route('/<any(user, c):typ>/<user>/') doesn't work,
probably because it then doesn't override the hardcoded rule in
youtube/.