Hide main 'Add' button if the form is already open

Currently the user can inadvertently open more than one 'Add account' form at the same time. I think we should suppress the main action button whenever we know it is redundant, for the following (subjective) reasons:

1. Multiple open forms make for bad UX. It becomes unnecessarily hard to keep track of which form is about what account, or rather: which of the multiple forms is still relevant.
2. Multiple open forms clutter up/foul up the intended navigation/UX in the app itself.
3. Multiple open forms eat up small but non-zero amounts of resources per page.

This MR proposes the dumbest thing that can still possibly work: maintaining a bool to track whether the add account form is still being displayed in the page stack somewhere.