`.File.Path` is OS dependant and uses `\` as directory separator. This leads to
cases where posts get the wrong URL, if in a sub folder, e.g.
`content/posts/folder\post.md`.
One idea was to replace `\` with `/` this however breaks valid files (on Unix
systems) which include `\` in the file name.
According to the docs [`path.Join`](layouts/partials/edit_post.html) is OS
unaware and converts to a Unix-like path.
> Note: All path elements on Windows are converted to slash ('/') separators.
PS: It looks like hugo also uses this internally to build URLs.
PPS: Solution was discussed in #278.
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