✨ [Feature request] - Add ability to toggle "Selected polities" and "Base map"

[jennyreddish]

Description of Improvement

On the world map, when you click on polities, their names and colours are shown in a box on the bottom right called "Selected Polities". Is it possible to add checkboxes that allow the user to toggle these and the base map to show only the polities they're interested in for a specific year? E.g. if I want to compare the Parthian Empire and Han Dynasty in 100 CE, I could select those and uncheck the base map, giving a clean look. Not a priority by any means, but might be a nice feature!

Dependencies

No response

Technical Notes

TODO:

• Add minus buttons to Selected polities legend to remove highlights
• Clicking the minus button for "Base map" when "Clickable borders" is set on the Background map radio switches it to "Basic unlabelled"
• Add a new button called "Hide unselected" which removes all polities that aren't selected from the map
• Add another new button that does "Select all" which will highlight all polities (and add them all to the legend)

Definition of Done

• The feature has been developed on a feature branch.
• The feature branch has been tested in the Staging website.
• A pull request has been created for the feature branch to be merged into the dev branch.