how to allow user to add html and javascript in my rails app

I want to create a Rails app where users can create their blog in a subdomain like : user1blog.myapp.com, user2blog.myapp.com etc...

I want to allow users to add html and javascript (sometimes they need to add popups to their blog, or tracking code ...)

I see a lot of website which allow users to add scripts and html without any restriction, but How i can do that too without affecting the security of my Rails app ?