Refactor unique comment to processor, rephrase specs in untriaged_helper

What does this MR do and why?

This MR addresses some unresolved comment from !1549 (merged)

• !1549 (comment 1059030032)
• !1549 (comment 1059030028)

Also refactors unique_comment into the processor class so we don't repeat in all the sub classes.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review