[#1635] Fix repeat email sending

The `since` date wasn't being saved after sending an email to a user,
resulting in repeat emails being sent about the same activity.

There is a test for this, but for some reason it was working in the
tests, just not in production. Don't know why. This seems to fix it in
production.

ckan/lib/email_notifications.py

@@ -134,7 +134,9 @@ def get_and_send_notifications_for_user(user):
     # FIXME: We are accessing model from lib here but I'm not sure what
     # else to do unless we add a update_email_last_sent()
     # logic function which would only be needed by this lib.
-    model.Dashboard.get(user['id']).email_last_sent = datetime.datetime.now()
+    dash = model.Dashboard.get(user['id'])
+    dash.email_last_sent = datetime.datetime.now()
+    model.repo.commit()
 
 
 def get_and_send_notifications_for_all_users():