Allow `redirects.json` to work with or without trailing slash

[yndajas]

When debugging an issue with a couple of redirects on #1101, I found that they weren't working because I'd not put trailing slashes in the to property of the object in redirects.json. Ideally redirects.js would look for a match for window.location with or without the trailing slash (this should be an easy fix)

We could just document the requirement for redirects.json to match window.location exactly, trailing slash or not, but it would be better not to have to worry about whether the slash exists in a given context