[IMP] Add classnames to snippets body and replace mb*, mt* from snippet ...

...body's root by proper styling on the classes

Backport of 7f07d10 and 6adcccd with class names modified according to our discussion with al this morning.

After we have merged this we can delete the odoo-dev/odoo-design repository.

However, I need to check with chm if its snippet api really requires the classes like o_block_banner, especially regarding the new new snippet api introduced with 4734cba, as those class names are confusing with respect the new ones.