Resolve "Extract logic of who should receive notification into separate classes"

What does this MR do?

Solve the problems mentioned in #27477 (closed) :

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...

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Reduce technical debt.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• [ ] Squashed related commits together

What are the relevant issue numbers?

Closes #27477 (closed)