[Behavior] Auto-assign maintainer to new issues

[benbalter]

Another behavior of @jekyllbot (that's relatively new), when an issue comes in and @mentions an affinity team, one of the team captains are randomly assigned the issue. They're obviously free to unassigned or change assignment, but it creates a sense of distributed ownership of issues, based on where they are in the codebase.

[bkeepers]

After #13, here's how this could potentially be implemented using pre-built actions (which aren't implemented yet):

  - on:
      - issues.created
      - pull_request.created
      - issues.labeled
      - pull_request.labeled
    when:
      label: security
    then:
      # Assign a specific person
      assign: benbalter

      # or: Assign a random maintainer with commit access
      assign:
        random: true

      # or: Assign a random person from a team
      assign:
        random:
          from_team: security-first-responders

      # or: Assign a random person from a file
      assign:
        random:
          from_file: .github/maintainers.yml

[bkeepers]

After #90, this should be implemented as a separate plugin. Please do investigate.