Fix team gets cleared when user is checked in

[sezanzeb]

• findUsersByIDs loads team and teamRequest now (fixes the problem in the title)
• admitting and such do partial updates now, to avoid accidentally overwriting team and teamRequest (fixes the problem in the title too)
• add tests
• Removed yarn because it was removed from node:alpine it seems, so we would have been required to create another dockerfile for local docker development. Since I am incredibly tired of breaking changes, deprecations, etc. in javascript toolings and wasting my time changing config files to fix javascript projects, vanilla npm it is. Which means I'm changing configurations once again.
• Removed the small and broken frontend test, so that I can get rid of the enzyme dependencies that were incompatible with our react version
• Had to upgrade chart.js and fix breaking changes after switching to npm, because npm tells me that there are issues in the dependency tree