Replace CoffeeScript files

[MrSerth]

CoffeeScript (and especially the Rails integration) is no longer maintained actively. For the coffee-rails gem, the last release is from 2019. Therefore, we should replace all CoffeeScript files.

We could use this opportunity to clean the files if necessary. Otherwise, we should just decaffeinate them.

Potential tool support:

• https://decaffeinate-project.org/
• https://decaffeinate-project.org/repl
• https://awsm-tools.com/coffeescript-to-javascript

Things to consider:

• Similar to Ruby, CoffeeScript has an implicit return. We should check where return values are actually needed and remove the explicit returns (added by converters) otherwise.