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.
In the same fashion Newsletters is managed, with the only difference that
the preview is using the notification partial in the same way the index
of notifications.
Add new routes for the proposal notifications edition and
abilities to let moderators edit it (mark as ignored, hide, etc.).
The notifications are not flaggable because they doesn't work like that,
but in a similar way. The moderator/administrator is in charge of hidding
them through the UI, so the normal users don't flag it as inappropriate.
New controller Moderation::ProposalNotification to manage the moderators
work.
