Decaffeinate smallest .coffee files & init TODO list

[Splines]

See the coffee.TODO for the status. Let's break this down into multiple small PRs starting with this one that does the first steps to decaffeinate our codebase entirely, i.e. getting rid of .coffee files and replacing them with .js or .js.erb files.

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As specificed in the mentioned TODO file:

Everywhere where I don't specify something in parentheses, I've manually checked
that the new new .js or .js.erb file is called (via console.log) and that it works.

so to save some time, I don't expect reviewers to test everything manually (it really takes quite some time to do this). Maybe just read carefully through the replacements, compare them and then go on. Double-check entries marked with ⚠️

[f-buerckel]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.27%. Comparing base (da59363) to head (e480f08).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #732   +/-   ##
=======================================
  Coverage   54.27%   54.27%           
=======================================
  Files         170      170           
  Lines        7344     7344           
=======================================
  Hits         3986     3986           
  Misses       3358     3358           

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