[FIX] website, *: review scroll button and height option behaviors

*: website_blog

- The scroll button option is now again available for all snippets, not
  only when the height is forced to 50% or 100%. Indeed, nobody would
  think to change that option if the snippet is already naturally tall
  and you could well want that scroll button on small-height covers.

- The scroll button is now placed on top of the snippet content instead
  of below in its natural flow. This allows it to work with snippets
  like "Big Boxes" and actually prevent to miscenter the snippet content
  just by adding that scroll button.

- Review the option UI and allow to control the spacing around the
  scroll button.

- the "Big Boxes" snippet was at last adapted to work well with 50% and
  100% height options.

- The 100% height option has now associated JS so that the height is
  computed to use 100% minus the size of the header.

Note: this may be backported to 13.2 where the option was introduced and
where those improvements may appear essential for the feature to not be
considered as buggy.

Part of https://github.com/odoo/odoo/pull/57764
task-2224404