[IMP] speed up load_menus() by using SVG icons instead of png

Reduces load_menus answer size by 32% (between 20kb and 200kb savings
for the initial loading of the backend, depending on the number of apps
installed). Support for SVG icons in the web client for menus/apps.

Reduced PNG icons for apps list (8 bits PNG instead of 24 as our icons
don't need more colors as they are flat designs)

closes odoo/odoo#84280

Related: odoo/enterprise#24200
Signed-off-by: Fabien Pinckaers <fp@odoo.com>