events: normalize referrers

[slint]

Usually, we only care about the referrer's host (e.g. google.com, scholar.google.com, some-uni-page.edu). This could be implemented as an event processor to normalize the referrer string.

Something to take into consideration is also the country domain for sites like Google. For example google.fr, google.it, etc. could possibly be normalized to just google.com.