Introduce a new LegalDisclaimerOnDirectionResource processors

This makes sure issues with the direction label gets a legal disclaimer prepended to their description whenever they're created or updated.

Note that there's currently no webhook for epics at the moment so this cannot be done for epics.

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

Closes #160.