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.
Yes unfortunately that is hardcoded against the provider name, which was previously set to "IP" regardless of the language.

Now that this is replaced with "Unknown", the translation can change. In french, it shows "FAI: Inconnu" for example. So do you see any way to change that `if` to make it work if the provider is unknown? I don't really know the datastructure of `visitor` :/

```php
{% if visitor.getColumn('provider') and visitor.getColumn('providerName')!='IP' %}
```