First pass at an issues list.

This is far from perfect and uses dummy data since rietveld doesn't have
an API for this yet, but this get's something in place to start iterating on.

I think in the final UI we'll want a drawer that drags out from the side.
That's especially mobile friendly. And we probably want to rethink how to
group the lists of issues. This first iteration just matches the current
rietveld groups.
Make selecting an issue in the issuelist actually load it.

This shouldn't be necessary, but there's something I'm not understanding
about how to make the two-way data-binding work, so using signals for now.