Provide option to limit upload size

[joar]

Due to the possibility to overload a server by sending it HUGE POST requests, we should provide an option to instance-owners to limit the possible media upload size.

Presumably, we should put this option in mediagoblin.ini.

We should also display this information to the end-user by providing a notice in mediagoblin/submit/forms.py.

Here's a handy snippet provided by cwebber/paroneayea that might aid anyone pursuing this ticket's completion: http://wiki.pylonshq.com/display/pylonscookbook/A+Better+Way+To+Limit+File+Upload+Size