- Models: remove toplevel field and replace it with placeholder field for...

- Models: remove toplevel field and replace it with placeholder field for children, adding tagging option for children
- Migrations: change 0002_add_bug_report.py according to models.py
- Forms: add default placeholder for first field, remove queryset filtering according to toplevel attribute
- Admin: remove displaying of toplevel and replacing it by placeholder, add tagging
- Views/URLs: replace rebus_get_icon AJAX request with a new rebus_get_next_properties which also includes the children's placeholder and tagging possibility
- rebus.html template: remove default bug category labels, change javascript updating of next field according to new AJAX request, add functionality to toggle the ability to use tagging input and to change placeholder dynamically