We're hitting some crashes on the demo server from bots accessing the

bug redirect view for a review request that does not have an associated

repository. This view made an assumption that the repository exists,

which is generally fine for normal usage but fails when crawlers try to

be clever.
This change adds some bulletproofing to the bug-related views to

validate that the repository exists and return a 404 if it doesn't.

Ran unit tests.