Introduce incidents and test_cases resources

This was originally suggested at #228 (comment 776589859)

For now in order to query against incidents, one would do:

resource_rules:
  issues:
    rules:
      - name: Query incidents
        conditions:
          issue_type: incident
        action:
          comment: |
           Comment

It would be nice if we can use incidents as the resources instead:

resource_rules:
  incidents:
    rules:
      - name: Query incidents
        action:
          comment: |
           Comment

If we're doing that, we can also make it by default for issues as the resources, put issue_type: issue by default so that it's a less surprising behaviour.