[FIX] web_editor: force the grid images maximum width to 100%

[sobo-odoo]

In some theme customizations, images were replaced by new ones without specifying the usual image classes img img-fluid mx-auto. While two of them could be ignored, this is not the case of img-fluid, which limits the maximum width of an image to 100%. Without it, grid images in mobile view are overflowing, because the rule forcing their width to 100% has been moved in commit 1, to be applied on the desktop view only.

This commit fixes this issue by adding a rule for grid images, limiting their width to 100%. The problematic themes will also be fixed in the associated design PR.

[robodoo]