[FIX] website: fix the vanishing arrows of the image gallery snippet

Steps to reproduce the bug:
- Add an Image Gallery snippet on a the page.
- Stay in edit mode and click on one of the arrows (left or right).
- Bug -> The arrow disappears.

This bug comes from the fact that the "oe_edited_link" class is added
to the anchor of the arrow when clicking on it. Because of the scss
related to this class, the position of the arrow is modified. This bug
is fixed by adding the "o_not_editable" class to the anchor of the
arrow. By doing so, the "oe_edited_link" is not added to the anchor of
an arrow when clicking on it.

task-3147271

closes odoo/odoo#112058

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

addons/website/views/snippets/s_image_gallery.xml

@@ -27,11 +27,11 @@
                         <i class="fa fa-chevron-right"/>
                     </li>
                 </ul>
-                <a class="carousel-control-prev o_we_no_overlay" href="#slideshow_sample" data-slide="prev" aria-label="Previous" title="Previous">
+                <a class="carousel-control-prev o_we_no_overlay o_not_editable" href="#slideshow_sample" data-slide="prev" aria-label="Previous" title="Previous">
                     <span class="fa fa-chevron-left fa-2x text-white"/>
                     <span class="sr-only">Previous</span>
                 </a>
-                <a class="carousel-control-next o_we_no_overlay" href="#slideshow_sample" data-slide="next" aria-label="Next" title="Next">
+                <a class="carousel-control-next o_we_no_overlay o_not_editable" href="#slideshow_sample" data-slide="next" aria-label="Next" title="Next">
                     <span class="fa fa-chevron-right fa-2x text-white"/>
                     <span class="sr-only">Next</span>
                 </a>