Improve deletion of databases from Django administration page

The current behaviour when deleting a database from Django administration page is as follows:

• Deletion of the 'Database' entry
• Deletion of the 'Database protocol' entries
• Deletion of the 'Database set' entries

Since adding a database may also imply adding a 'Database set template' and 'Database output's, it would be better to remove these entries as well, when they are not used by any other databases installed on the platform.