[FIX] web_editor: ensure editor in iframe can be scrolled on crop image

The overlay around an image when cropping it is positioned absolutely.
If it has no relative ancestor, it will cover the entire body. This is
what happend in mass_mailing, making it impossible to scroll the page
while cropping. This commit addresses the issue by positioning that
editable relatively so that the overlay never goes beyond its bounds.

closes odoo/odoo#69878

X-original-commit: c4d423640a9f1f9892e9d24dc08531ba0bbf8887
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/scss/wysiwyg_iframe.scss

@@ -17,6 +17,10 @@ body.o_in_iframe {
     background-color: $o-view-background-color;
     display: flex;
 
+    .o_editable {
+        position: relative;
+    }
+
     #oe_snippets {
         top: 0;
     }