Unable to filter by 'pull requests' + 'untriaged'

[brynwhyman]

Attempting to filter by PRs and those that are untriaged provides results that are already triaged: https://silverstripe-github-issues.now.sh/?type=pr&mode=untriaged#

We've making a push to triage all pull requests as they come through to help our team prioritise review efforts. This view would be the triage queue.