* 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.
Instead of requiring views like the dashboard to know about plugins so
they can include their data in the context, this allows plugins to
define a mapping between a view and a function where the function
returns a dictionary of new context for the view. Each view would have
to purposefully enable this additional context before it could be used.
This will allow new content to be added to the pages without updating
the core with a combination of a plugin to add new context, and a theme
override of that page to use the new context.
