Some system emails need to be manually reviewed before being sent. This
new action allows admins to see a preview of all Pending to be sent
Proposal Notification Digest messages.
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.
