If given a string, tries to load its json content
and then continue using the base class.

And normalize custom fields imports/use.

This includes both save and to_representation.

Also improves coverage, code related change tests were not
properly done.

Also improves coverage, code related change tests were not
properly done.

This one will be used for put request and the
modification serializer will be built on top of it.

[common][api] Implement get_object in place of get_queryset for RetrieveUpdateDestroyContributionView

This view shall act on one object only and the check
should already be done there rather than in furhter
methods below.

It also follows the URL routes cleanup

These endpoints usually gives access to the "latest" object version
which is not clear.

If this is really something we want to provide it should be using
an explicit route ending for example with "latest" at the end.