Groups with unicode characters in the name do not load

[tnajdek]

The culprit is the slugify function which attempts to mirror names that server provides. This needs to allow unicode characters.

https://forums.zotero.org/discussion/comment/467555/#Comment_467555

[tnajdek]

So I've made a tweak to allow all letters from all languages for now which should resolve the reported issue. In truth, web library does not use this slug for anything so it could be anything, however it cannot be an empty string because routing expects there to be a group name in the URL.

This unfortunately does not resolve all problems. One can have a private group named "☃︎" which still produces empty slug, so we either need to disallow names like this or rethink the URL scheme.