UI: Fix addon URL escaping in manager

[ndelangen]

The URL injected into the manager for addons were not escaped properly causing them to 404 on some hosting platforms.

I added escaping for these URLs.

[ndelangen]

Hmm I'm having second thoughts on this remapping to different files thing...
If there are split bundles that might break the references..

Perhaps all I need to do is escape the URL? @tmeasday you said that "the files are there"..
Could you send me a zip file of that broken storybook as it was deployed?

Maybe I can make it work by tweaking the html a bit.

[ndelangen]

OK, I think if found a much better solution!

[tmeasday]

LGTM apart from typo in function name

[IanVS]

@ndelangen I think this change might have broken Windows support. See #19534. Or maybe not, but something in alpha.37 caused it to break, and this looks suspicious. Maybe by encoding backslashes, we're not able to convert it to a posix path later on using slash().

[mjcarsjens]

slash() is definitely not built to handle URI encoded backslash characters. Would there be any possible issues with just splitting on both / and \\ and then joining with / in sanitizePath()? This could theoretically break usage of \ in UNIX file paths, but I don't really see a use-case for this (even disregarding the fact that this would be a terrible idea wrt cross-platform usage) .