Improve UX in single-checklist camps

In camps with only a single checklist (which is about a 2/3 majority of all camps which have any checklists), we could improve the UX in several places.

- When editing a checklist content node in an activity, the checklist(s) are collapsed every time the modal is opened. If there is only one checklist, the modal could be open by default.
- When visiting the /overview/checklists view from the main menu, one has to select the name of the checklist first, before seeing the checklist items with their respective connected activities. If there is only one checklist, we could automatically navigate to that detailed view.
- In the main menu, the "Checklists" menu entry could be named after the sole checklist, if there is only one.