TNL-4781, TNL-4782

- Icon images can be uploaded from the django admin
- Test coverage improved

- Icon images can be uploaded from the django admin
- Test coverage improved

These types of fields do not need to be included, and attempting
to do so causes everything to break.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Add a django-rest-framework APIView that supports reading/writing the current value of a configuration model

Also remove useless docstrings where they were added to keep pylint
quiet.

Also, I replaced a number of tearDown methods with addCleanup instead.
And also remove some unneeded patch.stopall() calls.

Make donations configurable

Added donation button to dashboard

Generalize merchant defined data for payment processor

ConfigurationModels can be managed using the admin site. They are
append-only, and track the user who is making the change, and the time
that the change was made. The configuration is stored in the database,
and cached for performance.

[LMS-1220]