Fix the gitweb URL format in the docs.

The example for a raw file URL using gitweb said to use hb=<revision>, but the
correct format is h=<revision>

Fixes bug 3027.
Tell users to back up their database before upgrading their sites.

We do this when we're answering people's questions on the mailing list, so we
should probably do so in the docs as well.
Fix ValueError when the user enters a bad cache host.

If the user enters a cache hostname which doesn't split properly, it makes the
admin site unavailable. This is because the cache stats code was just blindly
trying to unpack the values without any error handling. I've updated it to
cache this case, log, and continue.

Fixes bug 3049.
Reviewed at https://reviews.reviewboard.org/r/4402/