Warn users about cancelled sections

[maximz]

I returned from the summer and noticed that a cancelled precept was still in my ReCal schedule. It's a phantom precept now, because I cannot remove it and reselect it. But we should warn users explicitly that something in their schedule has changed and they must revise.

Potential design cues for this:

• Recolor that entry with a warning icon
• Notification in top bar of ReCal
• email notification? probably not.