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.

app/controllers/accounts/logos_controller.rb

@@ -0,0 +1,10 @@
+class Accounts::LogosController < ApplicationController
+  def show
+    @account = Current.family.accounts.find(params[:account_id])
+    render_placeholder
+  end
+
+  def render_placeholder
+    render formats: :svg
+  end
+end