[FIX] web_editor: fallback on URL search when shape not found from slug

Since [1] illustration shapes are only obtained from their slug. Before
they were obtained from their URL.

After this commit the old behavior is restored as a fallback in case the
illustration shape cannot be found from its slug.
This is needed to allow importing shapes into the system from data
files.

[1]: https://github.com/odoo/odoo/commit/bde8abcfeb57c74438943e44215a7c8cb822329f



task-2793073

closes odoo/odoo#86858

X-original-commit: 91f2a989ddd7961f92a377b1bf74cc741dcbd3d0
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>