Improve virtual rooms

Use One2One relationship instead of inheritance. This allows to add and also remove the virtual features of a room

• Introduce new model
• Create migrations to migrate from existing to new model + a squashed version knowing only about the new model for fresh installations
• Adapt admin views
• Add django-betterforms as dependency to simply create a form allowing to generate rooms with optionally a virtual component in a single view and create that form
• Add view to create rooms in that view and use instead of default django creation form
• Use the new name/structure in templates
• Move RoomForm to forms.py to make imports easier
• Update batch creation of rooms

This resolves #150 (closed) and also resolves #179 (closed) since now rooms and virtual rooms (rooms with virtual features) are created using the same view