Add some site with statistics for admin

[Bozydarek]

Site for staff that shows some metrics like:

• no. of registered users vs no. of users that set their preferences vs no. of users that payment have been accepted
• no. of proposed lectures
• no. of free places in bus
• etc

Plus maybe some graphs

[Bozydarek]

For charts we can consider https://www.chartjs.org/docs/latest/

[Bozydarek]

Adding charts would allow us to visualize when most people register, which options are most popular and other things like that

[Bozydarek]

Simple example of using chart.js: https://github.com/ksiuwr/zosia16-site/commit/0a0cc07110b1828287c5d5d5cd3862393a12e64e

[Bozydarek]

This would be partly solved by #82, but most statistics charts (and other data) are still missing.

[Bozydarek]

For labels on chart we can use: https://chartjs-plugin-datalabels.netlify.com/guide/#table-of-contents