Rename values for type in NotificationRecipient to make it easier to differentiate them from notification_level
During our research for the cause of #56861 (closed) we were confused by the values used for
type. They were named in a similar fashion which confuses readers of the code and e.g. makes reasoning about methods like https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/notification_recipient.rb#L48 more difficult.
So we should evaluate new names for
type and maybe refactor the code in
NotificationRecipient so it is easier to find possible bugs.