[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
[ADD] website_less: added support for url type attachments linking images on disk
[IMP] website.snippet: snippets can be split into separate templates and be called into the list of the snippets

Backport of cd0522a
[IMP] website.snippet: add data-oe-call to call a template from the website and keep all content editable; used for demo page of snippets

Backport of 5c524ca