Resourceful routes for traces API

[AntonKhorev]

This is only for /api/0.6/gpx/. There are other trace-related routes such as /api/0.6/user/gpx_files that are not changed here.

A couple of caveats:

• We have a nonstandard trace metadata path gpx/:id/details since 01cfcbd. Later a standard path gpx/:id was added in 71f1554. Both path are currently supported. It's possible to convert details into a nested resource to give it a resourceful route. This will require keeping some copy-pasted code. How about instead we don't do that, keep the details path non-resourceful and deprecate it?
• We have a nonstandard create trace path gpx/create. A standard one would be just POST at gpx. Again it's possible to map gpx/create in a hacky way to the create method in the traces controller, or make a nested resource just for creating traces. How about instead we:
  • add a standard create path
  • keep the gpx/create as non-resourceful and deprecate it?

[gravitystorm]

Looks good to me, thanks!