Add prepend_mod to Registrations::CreateProject

What does this MR do and why?

Relates to: New users onboarding with hands-on labs

In order to replace LearnGitLab project template, we need to override the learn_gitlab_template_path method redict to jh own dictionary

Since the instnce method defined in included do block can't overwrite success, i need to remove it

eg:

require 'active_support'
module Foo
  extend ActiveSupport::Concern

  included do
    def puts_1
      puts 1
    end
  end
end

module Foo2
  extend ActiveSupport::Concern

  def puts_1
    puts 3
  end
end

class Bar
  include Foo
  include Foo2
end

pp Bar.ancestors #=> [[Bar, Foo2, Foo, Object, Kernel, BasicObject]]
puts Bar.new.puts_1 # return 1

eg2:

module Registrations::CreateProject
   ...
   
   included do 
     private
     def override_me
       puts 'override fail'
     end
   end
end
...

module JH
  module Registrations::CreateProject
    extend ::Gitlab::Utils::Override
    
    private
    
    override :override_me
    def override_me
      puts "override success"
    end
  end
end


Registrations::GroupsProjectsController.new.send(:override_me) # puts 'override fail'

cc @zhanglinjie @daveliu

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