Add separate legend for statistics charts

[charludo]

Short description

Remove the legend at the top of the chart, replacing it with a box in the right-hand column, as shown in the design proposal in preparation of #1097.

Proposed changes

• remove old legend
• generate new legend server-side, using jinja
• wire them together using the method recommended in the docs for chart.js

Side effects

• none

Resolved issues

Fixes: #3133

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Pull Request Review Guidelines

[JoeyStk]

Wonderful! Thank you very much! Looks very good :)

[lunars97]

Thank you for this PR! ❤️ Everything looks good except this space on the screenshot below between the flag and the language name. I tested it on Chrome Version 130.0.6723.58 and Firefox version 124

[JoeyStk]

Ah, one other thing: Since this builds on an already existing feature and is going to be a change that municipalities might notice, a release note could make sense? @osmers What do you think? :)

[osmers]

@JoeyStk yes, I think a release note would make sense :)

[charludo]

Thank you for this PR! ❤️ Everything looks good except this space on the screenshot below between the flag and the language name. I tested it on Chrome Version 130.0.6723.58 and Firefox version 124

Good point. Currently that sapce is reserved for the second language flag, but you are right, if no language has two flags enabled, it looks weird. Will fix.

[charludo]

Ah, one other thing: Since this builds on an already existing feature and is going to be a change that municipalities might notice, a release note could make sense? @osmers What do you think? :)

Ah right, forgot to add it! Thanks!

[lunars97]

Looks good now! 🚀

[JoeyStk]

Wonderful! Thank you :)