fixes #5780 - API v2 - RESTful way to add/remove many-to-many association one at a time rather than passing all id's

Note: Related to PR https://github.com/theforeman/foreman/pull/1773 to add auditing when adding/removing
I managed to break the condition with 
`curl -u admin:changeme localhost:5500/api/v2/organizations/1/links/users/5,UNKNOWN,9` 
getting the result 
`{error": {"message":"undefined method `to_i' for [\"5\", \"UNKNOWN\", \"9\"]:Array"}`. 
It could be improved to catch non-integers and return proper message.