Refactor discussions/notes code

What does this MR do?

Refactors code related to discussions/notes so we can easily re-use this code for Epics (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4997).

• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

Related to:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4997
• https://gitlab.com/gitlab-org/gitlab-ee/issues/3889
• It also (hopefully) fixes 42148