[FIX] web, web_editor, *: review chess-like bg for transparency preview

*: website

The style to achieve the chess-like bg result was duplicated at many
places. The 'web' app was also actually using an image from the
'web_editor' app (we can live with it in stable as web_editor always
comes with web in auto_install but this was still a mistake).

closes odoo/odoo#54154

Also: fix some layout issue in the colorpicker widget.
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>