[FIX] website, *: prevent bottom fixed elements from hiding buttons

*: web, website_livechat, website_sale_comparison

Before this commit, bottom fixed elements (e.g. the livechat button)
hid the buttons to go to the next step in website sale when these
buttons are located at the bottom of the page, on some screen sizes.

After this commit,

- A bottom fixed element is hidden if this element hides a button at
the bottom of the page.

- Bottom fixed elements are hidden if a modal is open
to prevent it to hide buttons or any important part of the modal.

task-2501400

closes odoo/odoo#69296

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