Add reaction to nudge approver to start a new pipeline

Add a reaction to nudge approver of a gitlab-org/gitlab merge request to start a new pipeline.

Screen_Recording_2021-07-02_at_4.14.39_PM

The reaction would create a discussion thread that needs to be resolved before merging:

At the moment, this will create a thread each time the merge request is approved by someone.

Closes #134 (closed)

Action items

• When adding a new Triager, verify that it has been added to the DEFAULT_TRIAGERS in triage/triage/handler.rb in the appropriate order.
• If applicable, identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review