Make guests unable to view MRs

## What does this MR do?

Make guests unable to view MRs. This also fixes a bug when a non-member user could get notification if mentioned (in private project, it's OK for public project).

Now if you are a guest and you will be mentioned in one of the MRs you won't get a notification. 

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/1410

See merge request !6673