Is your feature request related to a problem?

After #6110 was merged it is now possible to color/filter events in the category calendar per venue. This is not useful for organizations that have a single venue or few venues but that, instead, rely on room as the most relevant location-based information.

Describe the solution you'd like

It should be possible to color/filter events in the category calendar per room. I can imagine two ways to do this.

Option 1:
Add a "Room" as display option for the calendar legend. When "Room" is chosen, the legend displays a list of rooms. When more than one venue exists in the system, the rooms should appear grouped by venue. When rooms appear grouped by venue it should be possible to select all and clear all rooms under a venue with a single click.

Option 2:
Instead of adding "Room" as display option for the calendar legend, rename "Venue" to "Location". When "Location" is chosen, the legend follows the same logic as in Option 1. Additionally, it should be possible to collapse/expand the rooms under venues. This should be possible in two ways: 1) by clicking on the venue for collapsing/expanding the rooms under that particular venue and 2) by adding collapse/expand all venues control. When the venue is collapsed, the events in the calendar take the color of the venue instead of the rooms (since the rooms are no longer visible in the legend) and the venue itself becomes filterable.

Low fidelity UI:

▼ Venue 1 ☑︎
    Room 1 ☑︎
    Room 2 ☑︎
    Room 2 ☑︎
▼ Venue 2 ☒
    Room 1 ☐
    Room 2 ☑︎
    Room 3 ☐
▶ Venue 3 ☑︎

Describe alternatives you've considered

It may be useful to wait until there is a way to select/clear all the items in the legend (#6148) before implementing this. On the other hand, UX considerations for this implementation may inform decisions in (#6148).

Additional context

This feature request comes inspired by Perimeter Institute's in-house developed calendar view:

/cc @dwindibank