Add a trip-site-sector-spot on the fly

I could imagine a situation where we're in the field and discover we want to take a Field Day reading somewhere that doesn't match an existing trip-site-sector-spot.

Right now, I think we have to do that on the server side. It may be useful to do it in the field just like we do everything else.

One possible approach among many: "Add a site", for example, could be added to the site dropdown. (Adding a trip could be trickier.)