Add support for sorting search results by reverse date

This obviously only applies to mail search, but now lets you search
in a way to get the oldest possible hit first.

Request from Tom Lane
update apt.postgresql.org key with an extended lifetime till 2019 to cover
Ubuntu trusty per Christoph Berg
Give a 404 error when trying to load docs version 0.0

Noted by Josh Kupershmidt
Implement workaround for django bug #15152 for badly encoded URLs

Hopefully this will stop the system spamming us..
Do not allow admin users to modify a username after it has been created

This involves some changes to how the default Django UserAdmin is handled with
respect to saving the form, but we simply override the changes to keep all
of the default Django functionality intact, minus allowing a username to be
modified on edit.