Display uploaded file size sum vs upload limit

[ayleph]

There's a configuration option to set an upload limit, but there's no mechanism to display to a user how much of that limit is being used. I would like to add a display of the user's consumed upload limit viewable only by the user. Since it's possible to modify the upload limit on a per-user basis in the database, we should read the upload limit on a per-user basis instead of reading the global upload limit set in mediagoblin.ini.

For sites that haven't implemented an upload limit, I think it's still nice to show a user how much data they've uploaded. In this case, perhaps we could show something like "450 MB / Unlimited" or something similar.