On the proposal notification digest messages there was a link to each
notification. Instead we need that link to point towards the notifications
tab of the proposal.
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.
