Add interactive modal for leaving a space

In commit 026f16a, I left this note after having implement a basic space leaving feature:

Currently, leaving a space will auto-leave ALL rooms within that space.
In the future, we need to create a separate modal that allows the user
to selectively choose which rooms and or/subspaces will be left.
I think the best way to do that is a modal that behaves like a file copy
dialog when it encounters file naming conflicts, in which you can choose
whether to replace or skip each individual file, or you can replace all,
or you can replace none.
In the same vain, our LeaveSpaceModal should allow interactively staying in
or leaving each room, or leaving them all, or staying in all of them.

As part of this, once it is complete, we would also want to remove the LeaveSpace variant and LeaveSpaceResult action handling code from the JoinLeaveRoomModal.