Multiselect for territory filtering

[huumn]

This is related to #712 in that whichever is done first should be reusable in the other.

As with #712, we'll want to use something like https://react-select.com/home for the muliselect UI.

Effect: rather than visiting one territory at a time or being in home, you can muliselect multiple territories to view simultaneously creating an experience that resembles a temporary home.

The UI around this will be the trickiest part given that it'll need to fit in the navbar on all screen sizes. Hence, difficulty:medium-hard.