Add the ability to display customized just-in-time messages to display

[PeterGrace]

If you were to have a visitor, for instance, and you wanted it to say "Welcome Spacely Sprockets!" on your displays, but didn't have time to make a new web view for it, it would be useful if one could have a precreated announcement route on pi_director where we could store a short message in the Pi data structure. When there is data in that field, the 302 in /go/ would redirect the pi to the generic route and show the message.