[FIX] web_unsplash: accept premium image URL

"ERROR: Unknown Unsplash URL!" is generated when we try to add premium
Unsplash images. This is because when we add the unsplash image, it accepts the
image whose URL starts with "https://images.unsplash.com/" but the premium
image URL starts with "https://plus.unsplash.com/

".

This commit solves the above issue by checking that the premium splash
image link starts with the correct format.

sentry-4075507166

closes odoo/odoo#118147

Signed-off-by: Romain Derie (rde) <rde@odoo.com>