Avo 3 to Jumpstart Pro template

Happening

1. Add to the Gemfile. Possibly to show a choice for avo, pro or advanced. gem "avo", ">= 3.1"
2. Run bundle install
3. Add the route mount Avo::Engine, at: Avo.configuration.root_path
4. Copy the template files

Related PR to JSP that contains a few more changes that JSP apps need to accomodate Avo.

https://github.com/jumpstart-pro/jumpstart-pro-rails/pull/749/files