Use toggle_select action for legislation proposals

The `update` action is usually expected to behave the same way it does
everywhere else, which is updating a record using the `params` hash.

The name `toggle_select` comes from the name we use in a similar
situation for budget investments.

config/routes/admin.rb

@@ -194,7 +194,9 @@ namespace :admin do
   namespace :legislation do
     resources :processes do
       resources :questions
-      resources :proposals
+      resources :proposals do
+        member { patch :toggle_selection }
+      end
       resources :draft_versions
     end
   end