Mentat

Now only fields that are selected have a negation option ("not_detectors" etc.), but this should also be the case for fields with user input (such as Description). These fields should also be visible for all users and not just hidden in the URL.

Based on input from Pavla Hlučková.

Example use cases:
- Searching for events, that do not have "Test" category.
- Searching for events of particular event class (or category), that are from other detectors than "detector1". This can be useful when trying to correctly set parameters of an event class, because you want to see events from all different detectors that send events labelled with this event class.
- Searching for events that do not have a specific description.