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
@isratrade, helpers should be used for DRY-ing UI templates. Very classic example for this rule is the `audit_time` function, but the `owner_type` is actually part of the model (the audit object) as it exposes more semantics out of it.
The same goes for `audit_template?` and `audit_login?`.