Support default imagery for regions

[pnorman]

Right now there isn't a way to specify what imagery should be loaded by default for a region, preventing iD from appropriately selecting background imagery for users.

See openstreetmap/iD#1310

[jfirebaugh]

There's the default boolean property. It needs to be added to appropriate sources and used by iD. I'm assuming that a simple rule like "use the first default source that has a region specified, which region includes the current location, or fall back to Bing" will suffice.

[pnorman]

It needs to be added to appropriate sources and used by iD.

So we should be setting "default": true in cases where it is clearly the "best" imagery for the region. I suppose it's a simplification, but should handle many cases (e.g. South Africa, Nanaimo if I get their imagery, Surrey)