This is consistent to what we usually do. Also, we're applying the same
criteria mentioned in commit 72704d776:
> We're also making these actions idempotent, so sending many requests
> to the same action will get the same result, which wasn't the case
> with the `toggle` action. Although it's a low probability case, the
> `toggle` action could result in [selecting a proposal] when trying to
> [deselect] it if someone else has [deselected it] it between the time
> the page loaded and the time the admin clicked on the "[Selected]"
> button.
The button to select/deselect a proposal wasn't very intuitive; for
example, it wasn't obvious that pressing a button saying "selected"
would deselect the proposal.
So we're using a switch control, like we do to enable/disable features
since commit fabe97e50.
