Improve statistics

One-sentence description

Improve statistics

Changed behaviour

After merging !46 (merged) (ref #9 (closed)), we could improve the statistics page.

Suggestions:

• Add statistics about event attendance sorted by event type (from #389 (closed))
• Pizza chart. Showing percentages of everything > 5% with a max of 10 and everything else is 'other'
• Top 10 memberships of committees
• Percentage of active members compared to non-active members
• Bar chart with number of members per committees.
• Events heatmap with number of events per week

Also, some header on the page are unclear. As mentioned in #366 (closed).