FR: Highlight unplanned work in Kanban dashboard

[tstromberg]

Add a CSS label to highlight issues that were added after the milestone modification time -- for Kanban dashboards that are based on milestone.

This can be used to identify the addition of unplanned work, which some Kanban teams may have limits for.

[tejal29]

hmm I just saw this issue after submitting PR #269. This PR kind of tries to address this issues.
Is this what you had envisioned?