Add a trailing slash to the path element in the redirect source widget

The problem is that this trailing / *is* there on a non-multilingual site, so this would result in two traiing slashes and we need to make sure to prevent that.

That said, the language suffix is actually confusing too, because it's the language field that defines the prefix, so maybe we should just cut that off?

This is a patch that supports multilingual sites and has exactly one trailing slash.

The patch of @mizutaniDC (comment # 9) works perfectly for a multilingual site with path prefix.

I'm not sure about #9, #7 makes more sense to me, to never show the language prefix. Having the language prefix is misleading as you can then combine it with another language or language agnostic. Wondering if we should have a test for that.

RTBC for #7 - works perfectly for me and is language neutral at this page, which also makes more sense as the language selection is a setting at the bottom.

Committed #7.