cleanup the given url a bit

The way we concatenate things, we assume that (in _most_ configurations)
there's no / on the given url, so that we can construct urls like
https://roost-api.mit.edu/v1/zwrite.  So we strip one off if it's given.
We only strip one because we could imagine urls like
https://weird.configuration//v1/zwrite, and although no one current has
that problem...
Pass a context to backends, and use the config infrastructure for roost