Migrate to Mantine

Our current component library @vseth/components is based on an old version of reactstrap, which is incompatible with React 18. It's unlikely that @vseth/components will ever be updated to a newer version of reactstrap, since VSETH has moved to developing vseth-canine-ui, a Mantine-based component library. We thus have to move away from @vseth/components, since otherwise we'll not be able to upgrade to any future version of React / other related deps.

I've thus moved all our components to @mantine/core and vseth-canine-ui. This should unblock the React 18 upgrade and also unifies our development stack; eventmanager and comsol using the same frontend library should make it easier for contributors to get involved.

The current status of this MR is that I have migrated all components and @vseth/components is no longer a dependency. However, when starting this project I've decided to allow temporary visual regressions because I was unsure if the migration would be feasible. Thus, the MR is still a draft, with the following work items left to do:

• Discuss how much we should stick to the old design
• Fix all visual regressions. Verified pages:
  • category-page
  • document-page
  • exam-page
  • faq-page
  • feedback-page
  • home-page
  • login-page
  • modqueue-page
  • not-found-page
  • scoreboard-page
  • search-page
  • submittranscript-page
  • uploadpdf-page
  • userinfo-page
• In vseth-canine-ui, make a sticky header an option (we had a custom navbar before, but now it makes sense to use the VSETH one) This can be re-introduced later with the Mantine 7 upgrade, because AppShell is reworked there.
• Test parts where functionality was touched (e.g. Select)
• Format with prettier
• Decide what to do about weird warnings in eslint about react-keycloak (unmaintained project gives errors, fix isn't being merged). The warnings are from vseth-canine-ui, and will be gone with version 1.1.0.

After this MR is merged, various follow-up items can be tackled:

• Migrating to React 18
• Supporting a dark mode (this should be easy with Mantine, but we'll have to remove hard-coded values)
• Potentially making use of @mantine/forms etc. instead of our custom hooks