DigtalGlobe imagery -> make default

[bastiK]

Since 2017-05-09 we can trace from DigtalGlobe imagery: ​announcement

I checked a few places near the boundary polygons for high resolution Bing imagery (tagged hires:imagery=Bing) and compared with Mabbox as well. There seem to be places all around the earth where DigtalGlobe imagery is by far the best available, examples:

• ​https://api.openstreetmap.org/api/0.6/map?bbox=127.5973231,44.68525,127.6018667,44.6876718 - DigitalGlobe Standard
• ​https://api.openstreetmap.org/api/0.6/map?bbox=81.2251934,40.544732,81.2274653,40.5460263 - DigitalGlobe Premium

Now I don't really get the distinction between "Standard" and "Premium", sometimes one is better and sometimes the other. I guess we just make both default and let the user figure it out.