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.

spec/features/admin/system_emails_spec.rb

@@ -0,0 +1,43 @@
+require 'rails_helper'
+
+feature "System Emails" do
+
+  background do
+    admin = create(:administrator)
+    login_as(admin.user)
+  end
+
+  context "Index" do
+    scenario "Lists all system emails with correct actions" do
+      visit admin_system_emails_path
+
+      within('#proposal_notification_digest') do
+        expect(page).to have_link('View')
+      end
+    end
+  end
+
+  context "View" do
+    scenario "#proposal_notification_digest" do
+      proposal_a = create(:proposal, title: 'Proposal A')
+      proposal_b = create(:proposal, title: 'Proposal B')
+      proposal_notification_a = create(:proposal_notification, proposal: proposal_a,
+                                                               title: 'Proposal A Title',
+                                                               body: 'Proposal A Notification Body')
+      proposal_notification_b = create(:proposal_notification, proposal: proposal_b,
+                                                               title: 'Proposal B Title',
+                                                               body: 'Proposal B Notification Body')
+      notification_a = create(:notification, notifiable: proposal_notification_a)
+      notification_b = create(:notification, notifiable: proposal_notification_b)
+
+      visit admin_system_email_view_path('proposal_notification_digest')
+
+      expect(page).to have_content('Proposal notifications in')
+      expect(page).to have_link('Proposal A Title', href: notification_url(notification_a))
+      expect(page).to have_link('Proposal B Title', href: notification_url(notification_b))
+      expect(page).to have_content('Proposal A Notification Body')
+      expect(page).to have_content('Proposal B Notification Body')
+    end
+  end
+
+end