[REF] website: call _post_copy with website in the context directly

Before this commit, the website was given as a parameter of the
_post_copy function which is called after theme install. It was then
transfered through the context when calling the theme sub' post_copy
function.

It makes actually more sense to directly call the _post_copy function
with the right website in the context instead of a parameter. This will
also allow to call enable_view/disable_view in the default post_copy
common to all theme, with the right website.

closes odoo/odoo#41312

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