all: use the "static" paths for media

Since Django 1.4, we are not supposed to serve our CSS from "media"
folders, but from the "static" ones. Reason is, "media" folders will
contain files any user could have uploaded.

Adapt settings and paths to reflect that.
views: log the exception, if processing the upload doesn't work
views: use django.contrib.staticfiles finder to resolve paths

Just like "staticfiles" does serve the CSS files, it is the only algo
that knows how to put them back.
backfire.js: fix the regex to match 2-level CSS declarations

Modern browsers support @media styles, like:

@media screen
  {
  p.test {font-family:verdana,sans-serif;font-size:14px;}
  }

http://www.w3schools.com/css/css_mediatypes.asp

Which would break the old regex parser.
backfire.js: replace alert() with some console warning

It was too noisy, repeat pressing "OK" in the message box is annoying.