reuse more werkzeug code

[ThomasWaldmann]

werkzeug is (via flask) one of bepasty's main requirements anyway, so we should reuse code from there if possible.

e.g.:

we have code to parse http range header (maybe also other headers), but werkzeug already has this:

https://werkzeug.palletsprojects.com/en/1.0.x/http/#header-parsing