chore: refactor to use function components

[JannikStreek]

Further Notes

With phoenix 1.7 release, the new standard is to use functional components instead of views. Using functional components in live views was apparently already pretty standard before, but now it's possible to use functional components across the whole app. As this is the new default, we should refactor mindwendel to use this. However, the old views are not deprecated and can still be used. But all generators etc now use the new functional components as default.

• See also https://www.phoenixframework.org/blog/phoenix-1.7-final-released , "Unified function components across Controllers and LiveViews"
• See also https://hexdocs.pm/phoenix/components.html for some details in the documentation.
• Some more details on how to write a more complex component https://curiosum.com/blog/phoenix-component
• Verified routes: https://elixirforum.com/t/phoenix-1-7-0-final-is-out/54113

A good idea would be to break this up in at least 2 steps: Refactoring the controllers/views and second step, refactor live views.