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/controllers/admin/proposals_controller.rb

@@ -19,9 +19,16 @@ class Admin::ProposalsController < Admin::BaseController
     end
   end
 
-  def toggle_selection
-    @proposal.toggle :selected
-    @proposal.save!
+  def select
+    @proposal.update!(selected: true)
+
+    render :toggle_selection
+  end
+
+  def deselect
+    @proposal.update!(selected: false)
+
+    render :toggle_selection
   end
 
   private