[Superagencies][2/n] Add toggle for child agencies in Set Up Metrics

[lilidworkin]

Now that we're saving the right data in Agency Store, no matter where we are in the app, if the user is logged into to a superagency, we should know which child agencies belong to that superagency.

We'd like to add a dropdown on the Set Up Metrics page that looks like this Figma.

The dropdown should only appear if the current agency is a superagency, and it should be populated with that agency's child agencies in alphabetical order. It should also be searchable.

When the user selects a child agencies, the page should behave similar to what would happen if the user had used the menu in the top left to switch to that child agency (but only this page is refreshed).

If the child agencies differ in the systems they belong to -- for instance if some of the child agencies belong to Law Enforcement and some to Courts -- we want to add sections to the dropdown and group the child agencies by which systems they belong to. They should be sorted alphabetically within each section. Reach out to @mxosman and @forrestkim00 when you get to this part for more clarity!