The spec for 'participatns' notifications in notification_service for both
issue and merge_request is pretty simple but it was fully copied without sagnificant changes
8 times (!!!) for EVERY situation you need to notify this group of user which caused to file
to extra growth and not being very DRY. And I love DRY. Since the spec is already too messy and
most likely gonna to increase the size nearest time, I decided to refactor those parts.
