Remove ~"pipeline:mr-approved" from MR when there are no approvals

Context

Closes #1487 (closed)

What does this MR do and why?

Two refactorings were done to extract the functionality I needed to implement the new processor from other processors.

• [Refactoring] Extract the NeedMrApprovedLabel class (commit)
• [Refactoring] Extract merge_request_approvers method (commit)
• [Feature] Add new processor to manage the MR approved label (commit)

Expected impact & dry-runs

Apart from removing the pipeline:mr-approved label when approvals are removed, no impact is expected, but since this MR is refactoring some core aspects of the merging process (trigger new pipeline after adding the pipeline:mr-approved label), we should monitor/test it.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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