[FIX] website: fix image carousel

We fix a number of problems:
    - grayscale filter not being applied except on desktop Firefox
    - incorrect vertical-align on iOS
    - image sliders being displayed below the margin on iOS
    - carousel indicators flickering at initial load
    - very short indicator panel, almost unusable on mobile
    - misaligned indicators because of first margin
There are two modes on which these fixes apply:
    - image gallery in slideshow
    - slideshow in modal
       (e.g. obtained by clicking on the gallery set on masonry mode)
Note that due to a refactoring, the gallerySlider was not loaded in the modal,
completely breaking the indicators.

Coauthored by @nle-odoo

opw 1874274

closes odoo/odoo#27226