UK/ME: add LastModifiedMixin
UK:/ME use LastModifiedMixin
ME: added 'LastModifiedMixin' class to these viewsets
ME: modified tests; removed last-modified asserts where they aren't required
ME: added date_modified property to UserProfile model
ME: checking if 'obj' is None then giving it a default value
ME: added migrations for the added date_modified property in UserProfile model
ME: reverted to initial changes and added tests to check 400 response hasn't last modified header
ME: removed 'last_modified_field' property
ME: removed 'last_modified_field' property
ME: removed 'last_modified_field' property
ME: return last modified header only if status code is less than 400
ME: refactored code; removed which data argument and check for UserProfile instance
ME: removed 'last_modified_field'
ME: code refactor to fix failing user viewset tests
Merge pull request #752 from onaio/744_last_modified_mixin

744 last modified mixin
UK: add media file itemsets.csv for select_one_external
UK: code refactor, docstrings