Improve analytics charts

[cs1m0n]

• time spent reading / number of articles read
• what books / publications are read frequently
• Create/Add to charts the use of dark mode (together with the other events, comparison light vs dark mode?)
• age + country groups
• search conversion rate (how many click on a search result among all who search)
• what is mostly searched for

[cs1m0n]

@karsten-kuepper #91 has been fixed, so you can now group users by age