Bugs 938723 948491 951515 - Curated group viewing and joining

Implement joining groups that might require approval.
Add through model to group membership so we can track membership status.

Fix bug 948491 - join group
Fix bug 938723 - view group as curator
Fix bug 951515 - curated group IRC channel and member count not displayed
Merge pull request #748 from caktus/dan-wray-join-groups-combined-branch

[Fix Bug 938723] Dan wray join groups combined branch
remove accidentally added setup.cfg
Bug 938680 - Don't show "Remove X" button for yourself

When a curator is viewing a group, the group members are shown
withe a "Remove X" button that lets the curator remove the user
from the group. Curators should not be able to remove themselves
from the groups they curate, though. So don't show a "Remove X"
button on a user's own member card when the user is a curator
viewing a group they curate.

Fix bug 938680
Merge pull request #755 from caktus/bug-938680-disallow-curator-removing-themself

Bug 938680 - Don't show "Remove X" button for yourself
[fix bug 956712] Add missing required class in registration's optin checkbox.
[fix bug 956734] Remove <h1> and </h1> from page title.
Merge pull request #756 from glogiotatidis/956712

[fix bug 956712] Add missing required class in registration's optin chec...
Merge pull request #757 from glogiotatidis/curatedgroups

[fix bug 956734] Remove <h1> and </h1> from page title.
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.