[ADD] website_less: compatibility layer for themes written in Less for website 8.0

Backport of website master to get the designer themes working on 8.0.
[IMP] website.theme: close the modal when the user clicks outside or presses escape

Backport of 96adeaa
[IMP] website.snippet: snippets can be split into separate templates and be called into the snippet list or in a demo page

Backport of 52ca425
[IMP] website_less: Add classnames to snippets body and remove mb*, mt* from snippet body's root

With classnames provided to snippets bodies, css should be use for the purpose of margins, allowing the overloading trough css for themes
Backport of 7f07d10 and 6adcccd
[IMP] website_less: fixed table_row while keeping footer extension feature

Designers couldn't use display: block without breaking the footer extension feature