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.

config/routes/admin.rb

@@ -32,7 +32,11 @@ namespace :admin do
   resources :debates, only: [:index, :show]
 
   resources :proposals, only: [:index, :show, :update] do
-    member { patch :toggle_selection }
+    member do
+      patch :select
+      patch :deselect
+    end
+
     resources :milestones, controller: "proposal_milestones"
     resources :progress_bars, except: :show, controller: "proposal_progress_bars"
   end
@@ -232,7 +236,10 @@ namespace :admin do
     resources :processes do
       resources :questions
       resources :proposals do
-        member { patch :toggle_selection }
+        member do
+          patch :select
+          patch :deselect
+        end
       end
       resources :draft_versions
       resources :milestones