Move actioncable javascript to app/javascript and change .coffee -> .js

- Rename action_cable/*.coffee -> *.js

- Move app/assets/javascripts/* -> app/javascript/*

- Rename action_cable.js.erb -> action_cable/index.js.erb

Renaming the extension to .js is in preparation for converting these
files from coffeescript to ES2015.

Moving the files to app/javascript and putting the entry point in
index.js.erb changes the structure of ActionCable's javascript to match
the structure of ActiveStorage's javascript.

(We are doing the file moving and renaming in a separate commit to
ensure that the git history of the files will be preserved - i.e. git
will track these as file renames rather than unrelated file
additions/deletions. In particular, git blame will still trace back to
the original authorship.)