CONNT-25: Removed Django admin customisation:

- Removed custom HTML template for Django Admin when adding a new
communication.
- Reimplemented limiting of source/destination to use filtering
information passed-in by default from Django Admin.

CONNT-25: Updating application and project to use Django 1.11.x:

- Bumped Django version in both the development requirements and setup
script to 1.11.x.
- Bumped Django Crispy Forms to version 1.7.0 in both development
requirements and setup script.
- Updated import of URL-related modules to use the new path (previous
one is deprecated).
- Added explicit on_delete = models.CASCADE option to all foreign key
fields (old implicit behaviour will be deprecated in Django 2.0).
- Fixed the custom change_list.html template used in Django Admin to
render without errors.