Improve domain splitting for "Hosted by" header

[luelista]

If a preferred web instance is set, an additional "Hosted by example.com" label is shown. This label is supposed to display only the main domain without subdomains, so if "element.example.com" is set, we show "Hosted by example.com". However, the logic doesn't do that at all (in only works by accident for domains where the subdomain is exactly one character longer than the TLD, matching the quite common chat.something.org or app.something.io structure).

The proposed code is still somewhat flawed as it wouldn't work properly for e.g. a chat.example.co.uk domain (displaying Hosted on co.uk), but at least it works for the currently trusted instances.