Add docstrings for every endpoint in backend

[lorenzschreyer]

The method headers should be uniformed and the classnames should be renamed to match others

e.g.

# 6. Delete Application
# https://github.com/ampcc/coding-challenge/wiki/API-Documentation-for-admin-functions#6-delete-application
# /api/admin/applications/{applicationId}

def delete(self, request, *args, **kwargs):
        """
        delete Application with
            query:
                applicationId
        """

[lorenzschreyer]

        """
        create Application with
            required arguments:
                applicationId

            optional arguments:
                challengeId
                expiry
        """

[lorenzschreyer]

not needed because of refactor