Set default external encoding to UTF-8.  #7.

This fix has been running in production for a couple of months now with
no evident ill effects, so I'm submitting it for permanent inclusion in
the tracker codebase.

The main source of external data in the tracker comes from its Warrior
clients and seesaw clients via its Redis instance.  Redis is
encoding-agnostic, and Warrior/Seesaw processes _usually_ run UTF-8.

I don't believe that this is a total fix; I don't think there's anything
stopping someone from sending e.g. a username encoded in UTF-16.  But
this will solve the common cause of #7, and we can build on top of this
work, e.g. enforcing UTF-8 for all Seesaw data returned to the tracker.
Merge pull request #9 from ArchiveTeam/expect-utf8

Set default external encoding to UTF-8.  #7.