DW: removed the xls_file as a reqired field in a form put method
Merge pull request #759 from onaio/750_xls_file_not_required_in_PUT

750 removed the xls_file as a reqired field in a form put method
UK/DW: sharing projects, role is also applied to project forms for user
DW: Owner cannot remove self from project if there is no other owner
DW: Corrected flake8 issues
DW: Checks if user is less or equal to one
ME/DW: Added remove user serializer
DW: Code refactor
DW: Code refactor
DW: Updated the doc string