[MERGE][REF] website_slides: allow / improve channel and slide creation

This merge provides new modal to create slide channel from the 'New' website
menu, like blogpost or forum. It also improves the upload slide widget from
channel homepage.

This merge also brings new slide type 'webpage'. This allow user to create
completely customized content as a slide.

This merge is linked to Task-1938643. More generally it is linked to the
eLearning feature [1]. For more details see subcommits.

[1] see task ID 1902304 (main eLearning task) PR #29876

closes odoo/odoo#30991