Extract logic of who should receive notification into separate classes

As noted in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6813#note_16937503:

all the logic related to who's watching something, or who should be notified, ideally should be placed in a separate class, not inside the service responsible for notifications

Also, methods name will need to be improved:

For example, this method doesn't return a setting, but returns users. And other methods that have users in the name actually return user_ids...