Thread locally scoped gettext

[Christopher Allan Webber]

Currently mediagoblin is not threadsafe, but really the only reason
for this is mg\_globals.gettext which is set at the start of every
request. So if there are two threads running in parallel these
might get clobbered.

However python has some thread-local scoping:
`http://docs.python.org/library/threading.html#threading.local <http://docs.python.org/library/threading.html#threading.local>`_

We could probably use that. SQLAlchemy has some thread-scoped
session, so theoretically it uses this, or we could look at what it
does use:
`http://www.sqlalchemy.org/docs/orm/session.html#sqlalchemy.orm.scoped\_session <http://www.sqlalchemy.org/docs/orm/session.html#sqlalchemy.orm.scoped_session>`_

Not super high priority as preforking does work fine-ish, but would
be very nice to have.