Reference external template bundle file on template import

• create a new trial project
• access studio
• a template is automatically imported
  => a separate bundle.html file is added for the template

problem is that anything brought by the template is not taken into account due to that. We should either merge its content into the project's bundle.html file, or reference its content like andré explained somewhere (probably better and cleaner I guess)
it's not only for this use case, it also applies to any external template you could import in your project

AC

• When I import an external template, its bundle file is referenced in the project's bundle.html file (bundle.html file is created if it does not exist already)
• When I import an external template then deploy my config, my project has the content from both my project and the template
• When I remove an external template, the reference to the bundle.html file is removed as well (this behavior will be improved later with NXS-4749 )