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