Before this change, `RemoveMemberModal` was mounted on the page
separately from the Vue app. `RemoveMemberModal` listened for
clicks of the remove member button and read data from the button's
HTML dataset.

This change moves `RemoveMemberModal` into the Vue app. Clicking
the remove member button now directly opens `RemoveMemberModal`.
`RemoveMemberModal` reads data from the Vue app's Vuex store,
in keeping with the app's state architecture.