user_groups: Use 'path_only=True' in REQ for 'user_group_id' in views.
user_groups: Use 'path_only=True' in REQ for 'user_group_id' in views.

'user_groups' endpoints are currently under 'pending_endpoints' in
test_openapi.py (even after being documented except one), due to the
'user_group_id' and 'group_id' parameter name mismatch in the
url config and the view functions.

This commit includes 'path_only=True' for 'user_group_id' parameter in
views to avoid the failure of 'test_openapi_arguments', in
test_openapi.py, which excludes the path parameters. This is a prep
commit for renaming 'group_id' to 'user_group_id' in the documentation
and removing the 'user_groups' endpoints from 'pending_endpoints'.

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