Bug 951840 - Send members of curated groups emails upon changes
Fix bug 951840

Changes from reviews:

* Use .get(q) instead of .filter(q).get()
* Change join_group to always add the logged-in user.
* Remove special privileges for superusers in joining and leaving groups.
* Change how we track which membership requests we've sent reminders for.
  Now keep timestamps on the group for when the last request was received,
  and when the last reminder was sent.
* Don't import from deprecated django.conf.urls.defaults.
* Add quotation marks around group names in emails.
* Use userprofile.full_name for user name
* Provide full link to group in email, including scheme and host
* Don't send 'rejected' email to someone when they cancel their pending membership request.
* Use PKs instead of time to track whether we need to send another reminder.