Improvements to webview

This improves the "Open issues" view by allowing each column to be collapsed and expanded. This required adding Javascript, so I introduced a directory for static content instead of adding another static file mapping.

It adds labels to the issue links on the "Open issues" view, so that it's possible to read issue descriptions without clicking through.

It adds explicit "scope" attributes to table headings on all views. I think this can help screen readers to present the tables properly, but I'm not sure.