Better UI for Checklist item selection in activities

Currently, you have to click on the area with the selected checklist items to change which items are selected:

When no item is selected yet, it just shows an "Edit" label:

Clicking this opens a hand-rolled modal dialog with accordions and switches:

This differs e.g. from LA thematic area, where we have a multiselect dropdown:

Some core team member wrote: "I'd prefer a "edit" button (or better "select items") button instead of clicking on the already selected items", referring to it being awkward to have no edit button and no caret to click when there are already some checklist items selected.

Extracted from [#6018](https://github.com/ecamp/ecamp3/issues/6018#issuecomment-5645148978)

Before this can be implemented, we need to decide how this interaction should work. Unlike the LA Thematic Area, we have accordions in the checklists because there can be multiple checklists with lots of items.

Start by comparing the current checklist selection interaction with the LA Thematic Area multiselect shown in the issue, paying particular attention to multiple checklists and their accordions. The first step is to decide and document how users open the selector and choose items; implementation can begin once that interaction is agreed and its completion criteria are clear.

Written by the indexing model from the issue text.