[FIX] website: restore footer slideout effect on Firefox

For some strange reason, the footer slideout effect was not working on
Firefox anymore. The height of the footer is not understood by Firefox
sticky implementation which it seems to not understand because of the
combination of 3 items: the footer is the last :visible element in the
`#wrapwrap`, the `#wrapwrap` uses flex layout and the `#wrapwrap` is the
element with a scrollbar.

As neither of the last two items can be changed, this commit fixes the
problem by adding a pixel div element in the DOM after the footer on
page load, if the slideout effect is enabled. This is obviously not
ideal but solves the problem while waiting for another solution.

Part of https://github.com/odoo/odoo/pull/60271


task-2312878

closes odoo/odoo#60271

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>