Use separate actions to select/deselect proposals

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.

app/components/admin/proposals/toggle_selection_component.rb

@@ -8,7 +8,11 @@ class Admin::Proposals::ToggleSelectionComponent < ApplicationComponent
   private
 
     def action
-      :toggle_selection
+      if selected?
+        :deselect
+      else
+        :select
+      end
     end
 
     def selected?