Follow-up from PEP MVC: Define SSoT for pipeline rejected by rules

The following discussion from !151361 (closed) should be addressed:

• @fabiopitino started a discussion: (+1 comment)

                    next if pipeline.rejected_by_rules?

  Follow-up: we should defined a SSoT method in Ci::Pipeline#rejected_by_rules? which delegates to Enums::Ci::Pipeline internally.

  • remove persistable_failure_reason? in favour of the a more intention revealing filtered_by_rules? method in the enum class.