[IMP] website: create special extension page redirect backend + own template

Before this commit:
1. You could create a page with supported extension in the path (eg: myfile.js)
You would then be redirected to the webeditor (myfile.js?enable_editor..)
But in this case, it would just redirect you to a plain text page without odoo
layout because supported extension are rendered with a specific mimetype.
(is it is a .js page, it will just render the page content as a js file)
2. These kind of page would still call t-layout that you should be removed in
backend in order to make it work

Now:
1. You will be automatically redirected to the back end to edit your file
instead of landing on a text-only page.
2. Supported extension now have their own template (eg: .js file now have
<script text=..> tag added automatically in their content)