Canonicalize problem notice urls

Previously the notice visible at apps/:id/problems/:id?notice=X
would change each time a new notice was reported because pagination was
using reverse ordering (notice=1 is most recent). This made it
problematic to show someone a particular notice via a URL.

Now, notices are indexed such that ?notice=1 will show the oldest
notice. This makes it so the content avaiable at that URL doesn't keep
changing every time a notice is reported.

leaving off the ?notice=X param still defaults to the most recent
notice.

This change simplifies the code, drops a (now-unused) helper method, and
makes errbit less surprising.