This feature was never used. Partly this is because it is very
complicated to actually use.

However I also doubt that there are many usecases. I don't think it
makes sense to filter on anything but the things that are already
supported (attributes, previous study participations, geo location).
So the only real usecase is to combine these filters in a more
complicated way than the UI allows.

In those rare cases I personally would opt to create a temporary
boolean attribute, automatically fill it with a script and then use the
regular filter UI. We could propose this as an alternative in the
changelog.

This is a security issue because users were able to select groups from
other studies, bypassing permission checks.

fixup 088f1659

-   matches the propose cross-service exchange format
-   removes study.study_mode redundancy
-   matches ParticipationRequest.status

fixes deprecation warning for django 3.1

notably filters

to align with CASTELLUM_EXPECTED_SUBJECT_FACTOR

instead of using messages

see also !509

to align with FilterGroupUpdateView

this is closer to how we refer to it