Add API endpoints for planetary bodies' textures

[IvanSanchez]

Feature request, related to #75.

The maps currently use image tiles from tiles.kerbalmaps.com, which is nice for the default bodies, but will not work for custom planetary bodies (Kopernicus, RSS, etc).

It would be nice if the Telemachus' web server could provide these images (along with basic planetary body info such as the biome names and orbital parameters). This would allow the maps to work seamlessly no matter the planetary body.

The KerbalGIS project does this, but was made for KSP 0.23 and most probably won't work, but some of its pieces and algorithms could be salvaged.

Otherwise, providing the body textures in .png format would be enough, even if that would add a bit more complexity to the map client.