[FIX] web_editor: fix images getting absolute URLs in grid mode

[1] introduced the grid mode and with it a util that reloads lazy
images in order to prevent a bug in Chrome which make images disappear.

This util read the .src property of the image element. That property is
the absolute URL.
In order to keep the exact URL that was on the img attribute, one needs
to use "getAttribute".

This commit does exactly that.

Steps to reproduce:
- Drop a Image - Text snippet
- Drag the image to enter grid mode
=> At the same time the image is reloaded with the wrong src

[1]: https://github.com/odoo/odoo/commit/cc406afcea7bf5846233a9f97a4a8ac5f618f3ec



closes odoo/odoo#134019

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