Add group_level? and instance_level? helpers to service.rb model

The following discussion from !57381 (merged) should be addressed:

• @arturoherrero started a discussion: (+2 comments)

  This is really nice! Finally, our vocabulary matches the code.

  I guess we should also add instance_level? and group_level? at some point. I was using: service.instance?, service.group_id or service.project? but this looks better.