Fixes #6101 Unknown providers are now shown as "Unknown" instead of "IP"

This implements the change described in #6101.

- [x] change from IP to `Unknown` (i.e. "IP" is now merged with real "Unknown" IPs)
- [x] in the report footer, we could write `Unknown provider means the IP could not be looked up. See <a href='http://piwik.org/faq/general/faq_52/'>this faq</a>.`
- [x] update [the faq](http://piwik.org/faq/general/faq_52/)
- [x] when `disableLink` is used, we could hide the link to piwik.org

I do not have the access to edit the FAQ.
maybe a  test equivalent to the old =='ip' could be: `visitor.getColumn('providerUrl') is not empty`