Add Preview Pending action to Admin System Emails

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.

config/locales/en/admin.yml

@@ -651,9 +651,14 @@ en:
         sent_guide: "This is how the users see the notification:"
         send_alert: Are you sure you want to send this notification to %{n} users?
     system_emails:
+      preview_pending:
+        action: Preview Pending
+        preview_of: Preview of %{name}
+        pending_to_be_sent: This is the content pending to be sent
       proposal_notification_digest:
         title: Proposal Notification Digest
         description: Gathers all proposal notifications for an user in a single message, to avoid too much emails.
+        preview_detail: Users will only recieve notifications from the proposals they are following
     emails_download:
       index:
         title: Emails download
@@ -1051,7 +1056,10 @@ en:
       show_image: Show image
       moderated_content: "Check the content moderated by the moderators, and confirm if the moderation has been done correctly."
       view: View
-      preview: Preview
+      proposal: Proposal
+      author: Author
+      content: Content
+      created_at: Created at
     spending_proposals:
       index:
         geozone_filter_all: All zones