[#1384] Add custom to_native method to Base64ImageField
[#1384] Add custom to_native method to Base64ImageField

Add Base64ImageField.to_native() to include the full path to the image
in any API use and add the option of requesting a thumbnail as part of an
API get request.

See docstring and comments in to_native() for more details.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr