Clearly describe price categories

[developeregrem]

The price classes must be described more clearly so that the calculation of the invoice amount can be made easier.
Just like the "always" option for days, an "always" option should also apply to the period (it is best to create a new attribute).
Depending on the origin, it must be checked whether there is already an active category that collides with the new one.
Concretely:

• For an origin, there can only be one active price category for identical appartment-specific values, period, valid days (check overlapping periods).
• Price categories with a time period have a higher value than categories that always apply (important for the later query which categories apply to a reservation).

If a collision has been detected, the user is shown a short list with the categories that collide.
Applies to creating and editing a price class.