Added a link field to the Membre model
Removed the "Communications" section from Seminari

Both the section in the template as well as the method on the class.
This is no longer needed as we will create a Communications theme from
now on.
Fixed 500 template to work without STATIC_URL
Cleaning up a couple of files
Presenters and staff can upload files to a Xerrada

This is done on a site page (not from the admin).