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

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

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