Add Admin System Emails view & first system email

Admins need to be able to see what a particular System Email looks like
with dummy data.

Also adding the first system email to be managed: the proposal
notification digest.

In case the current admin doesn't have any Notifications from
 ProposalNotifications there will be a crash. We'll solve this in later
 PR's with a system to "inject" sample text in email templates without
 having to generate records in the database.

config/routes/admin.rb

@@ -167,7 +167,6 @@ namespace :admin do
 
   resources :system_emails, only: [:index] do
     get :view
-    get :preview
   end
 
   resources :emails_download, only: :index do