Extract generation of js_code into module

[ashgaliyev]

This change is 

[coveralls]

Coverage remained the same at ?% when pulling 7a4207a on andrey/add-js-pro into 735c434 on master.

[coveralls]

Coverage remained the same at ?% when pulling c5668a8 on andrey/add-js-pro into b4d3763 on master.

[justin808]

Some suggestions.

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashgaliyev)

---

lib/react_on_rails/helper.rb, line 441 at r1 (raw file):

        initialize_redux_stores,
        react_component_name,
        render_options

         props_string: props_string(props).gsub("\u2028", '\u2028').gsub("\u2029", '\u2029'),
         rails_context: rails_context(server_side: true).to_json,
         redux_stores: initialize_redux_stores,
         react_component_name: react_component_name,
        render_options: render_options

---

lib/react_on_rails/server_rendering_js_code.rb, line 14 at r1 (raw file):

      end

      def server_rendering_component_js_code(

      def server_rendering_component_js_code(
         props_string: nil,
        rails_context: nil,
         redux_stores: nil,
         react_component_name: nil,
         render_options: nil
       )