Annotate step-by-step instructions with "[very] steep hill {up, down}" flags

[graue]

Currently, bikehopper-ui computes "steep hill" labels on the client side corresponding to each step in the instructions, but it would probably make more sense to compute this in the router along with the generation of those instructions.

(This is only a nice-to-have, because the client code, although hacky, is doing the trick for now.)