1386 receptionist

This adds a new sample group called "Receptionist". The idea is that this person should be able welcome subjects and find out which contact person (from which study) is responsible for this person, but nothing more.

I implemented this by adding a new permission called search_execution. This permission allows to bypass all checks in the execution search, including privacy level and study membership. On the other hand, this also means that this permission allows nothing more than that. If we would give the receptionist maximum privacy level and membership, this would have repercussions all over the place.

The UI is a bit awkward: Receptionist start with the "recruitment" and "execution" tiles, both of which contain empty lists. They have to go to "execution" -> "find subjects" in order to get to the only useful UI for them.

Code is also not that nice: I had to add the new permission to the recruitment app because execution does not have models itself. I set a custom property on ParticipationRequest to avoid duplicating permission logic in the template.

All in all I think this approach is not nice, but works and avoids adding too much complexity.