Refactor placeholder logo into common controller (#673)

This can be updated to redirect/pull from third party sources in future,
with the option of always falling back to the placeholder if there are
any failures.

config/routes.rb

@@ -35,6 +35,7 @@ Rails.application.routes.draw do
   resources :accounts, shallow: true do
     get :summary, on: :collection
     post :sync, on: :member
+    resource :logo, only: %i[show], module: :accounts
     resources :valuations
   end