Refactor CodeOcean::Config class

The new architecture memorizes settings (which we mostly did after reading the config so far) and also exposes the resulting file path as well as further settings.

This change is a prerequisite to define a dependency with Sprockets.

spec/lib/code_ocean/config_spec.rb

@@ -30,7 +30,7 @@ RSpec.describe CodeOcean::Config do
 
     context 'with a .yml.erb file' do
       let(:path) { Rails.root.join('config', "#{filename}.yml.erb") }
-      let(:read) { described_class.new(filename).read(erb: true) }
+      let(:read) { described_class.new(filename, erb: true).read }
 
       context 'when the file is present' do
         before { File.write(path, {Rails.env.to_s => content}.to_yaml) }