Fix #3111: Convert CoffeeScript to JavaScript

[mroderick]

This PR is a solution for #3111, it does a few things:

• Convert all .coffee to .js using CoffeeScript 1.12.2, which is the same version as coffee-rails is using (coffee-rails => coffee-script => coffee-script-source)
• Remove unused dependency coffee-rails

Customisations in custom/ui.coffee in userland will have to be converted to ``custom/ui.js` by users.

# something like this to convert customisations
npm install -g coffeescript@1.12.2
coffee --compile app/assets/javascripts/rails_admin/custom/ui.coffee`

Using decaffeinate was not possible, as the converted files would cause the rails_admin tests to fail (most likely due to it outputting ES6, and the Capybara tests using PhantomJS, which barely supports ES5).

[olleolleolle]

The build failure in 1 build target is:

Coverage (90.02%) is below the expected minimum coverage (90.05%).
SimpleCov failed with exit 2/home/travis/.rvm/rubies/jruby-9.2.6.0/bin/jruby -I/home/travis/build/sferik/rails_admin/vendor/bundle/jruby/2.5.0/gems/rspec-core-3.8.2/lib:/home/travis/build/sferik/rails_admin/vendor/bundle/jruby/2.5.0/gems/rspec-support-3.8.2/lib /home/travis/build/sferik/rails_admin/vendor/bundle/jruby/2.5.0/gems/rspec-core-3.8.2/exe/rspec --pattern spec/**{,/*/**}/*_spec.rb failed
The command "bundle exec rake" exited with 2.

No other target fails.

[olleolleolle]

When this has been merged, we can work on the CodeClimate issues presented – or do you want them to be handled in this same PR, @mshibuya ?

[mshibuya]

We can work on the issues later, thanks for the PR!