Define gemfile_path based on lib name for dummy app

Extensions developers need to make sure they add a non core lib name to
their Rakefile so that the proper path to Gemfile is generated in
boot.rb along with their dummy app.

Motivation here is help users boot their extensions using a preloader like zeus.

Fixes #4253
Use current_store helper on general_settings_controller
Moving current_store helper to Store concern to fix undefined method current_store API errors.