Slug generation is broken for most Unicode inputs

[mqp]

In our current implementation, when we try to generate slugs for hubs, scenes, and avatars, we generate them based on the name, but the slugification library we use barfs and returns invalid output for inputs that contain Unicode characters outside of a small whitelist. This causes a malformed SQL update, which causes the renaming operation to fail.

You can see someone complaining about the slugifier behavior here: h4cc/slugger#33

To the best of my understanding, it will fail if given as inputs any non-ASCII characters except the ones that are keys in this file, although there might be subtleties that I missed: https://github.com/h4cc/slugger/blob/master/lib/replacements.exs

I suggest killing our dependency on this slugification library and creating slugs by e.g. replacing non-URL-safe-characters in names with dashes.