Use separate actions to select/deselect investments

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 an investment] 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

@@ -66,7 +66,10 @@ namespace :admin do
     end
 
     resources :budget_investments, only: [:index, :show, :edit, :update] do
-      member { patch :toggle_selection }
+      member do
+        patch :select
+        patch :deselect
+      end
 
       resources :audits, only: :show, controller: "budget_investment_audits"
       resources :milestones, controller: "budget_investment_milestones"