[FIX] website: fix carousel arrows hidden with grid mode

Steps to reproduce the bug:
- Drag and drop a Carousel snippet onto the homepage.
- Add an image inside a Carousel slide.
- Set the "Content width" option of the slide to "Full".
- Set the "Layout" option of the slide to "Grid".
- Make sure the image touches the edge of the page.
- Bug: The arrows are below the image and are no longer accessible.

To fix this bug, we add 'isolation: isolate;' on the Carousel content to
create a new stacking context. This ensures that elements like
indicators and arrows will always appear on top of the carousel content.

task-3383061

closes odoo/odoo#127491

X-original-commit: 99133cafbb0cb0aa9563475eb364dfffc412d2fe
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -551,6 +551,9 @@ table.table_desc tr td {
     }
     // Content
     .carousel-inner {
+        // Create a new stacking context to ensure that elements like indicators
+        // and arrows will always appear on top of the carousel content.
+        isolation: isolate;
         overflow: hidden;
         height: 100%;
         .carousel-item {