Restore aerial map layer (when editing)

[philipn]

I had to disable the aerial map layer because it was causing the map editor to error out. The URL, http://ecn.dev.virtualearth.net/MapControl/mapcontrol.ashx?v=6.3 has been removed, after several years of deprecation. Microsoft is now using the bing.com domain for their maps.

Approaches to fix this:

1. Update olwidget and use whatever the new bing aerial map style is.
   a. Downside: we have major changes to olwidget, so this could be involved.

2. Migrate using the migration shim: https://github.com/rbrundritt/BingMapsV63ToV8Shim.
   b. Downside: we need to add that authentication bit, as the olwidget code doesn't do any authentication at the moment. Looks hairy.

3. Construct a Bing map layer a'la http://openlayers.org/en/master/examples/bing-maps.html. This is probably the easiest option.