* Moving plugins infrastructure to edx-django-utils
This PR extracts the code that enables plugins in edx-platform and puts it in edx-django-utils. This is done to allow other IDAS to add plugin functionality.
Some notes:
* The completion API was pulled out into a new repo, but left behind a
url() entry. That entry used a different namespace than the
pulled-out repo, so I had to fix the one place we used the namespace.
* Two urls couldn't be anchored because they broke tests:
url(r'users/(?P<user_id>\w+)/followed$', views.followed_threads, name='followed_threads'),
url(r'users/(?P<user_id>\w+)$', views.user_profile, name='user_profile'),
