Pass the cluster health alerts endpoint from backend to frontend

Move alerts-endpoint into the cluster_health_data helper so we don't have to append it in the haml file.

The following discussion from !13443 (merged) should be addressed:

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

  I don't have a strong preference I think this is fine considering that is going to be removed later, but should we move this to a helper file so it's easier to grok?

Prerequisites (must be completed first):

• https://gitlab.com/gitlab-org/gitlab-ee/issues/11879