Overhaul condition UI

[BurntimeX]

Conditions are used at various points in the software (e.g. automatic group assignment). Currently, all available conditions are listed in the form, resulting in a very bloated and confusing user interface. Especially when editing existing conditions, it is very difficult to find out which conditions are active.

Instead, only the active conditions should be listed in the form. A button can then be used to select and add further conditions from a dropdown. The same condition could also be set multiple times, which would allow us to omit the use of multi-selects for some conditions.