[WIP] Improve public URL handling

[jstcki]

@grossbart was complaining about that relative paths get turned into absolute ones by the current publicUrl default – the only way to prevent this was to set publicUrl to ".".

I tried to change this here but I'm not 100% convinced that this is the right approach or even have a super-clear idea on what it should be – there are many things to consider 😅.

My proposal would be to approach this change from the documentation side. That means we should write the documentation for how process.env.PUBLIC_URL, publicUrl, useBrowserHistory and basePath work or rather: how they should work. And then implement the correct behavior.

Edit: Actually, that documentation should probably be a guide about deployment in general rather than just documenting the configuration options.

Related #348 #350

[grossbart]

I added a bit of documentation about URLs and routing and think it's on the right track. But as expected, it revealed a conflict of how we want to handle routes in Catalog: in the first few examples I show relative and absolute URLs that I would expect Catalog to handle correctly, but technically it can't really. What would we want to use there?