Make messaging system testable

[Christopher Allan Webber]

We should make it possible to test the messaging system.

If you look at mediagoblin/utils.py you can see what happens when we _activate_testing() for some other things... ie, we insert template context info like so:

if TESTS_ENABLED:
    TEMPLATE_TEST_CONTEXT[template_path] = context

Maybe we should do something similar with messaging? Either we should store in a dictionary like TEMPLATE_TEST_CONTEXT, maybe TEST_MESSAGES (which gets wiped by clear_test_buckets()) or we should store in the session like request.session['test_messages'] (should also get wiped by clear_test_buckets())

I'm assigning this to Caleb Davis who I think is a good fit for it.