Before this commit, the /website/add was always redirecting the requests
to the newly created pages. This way, the controller could be called
from a frontend 404 page (when clicking on the "Create page" button),
and from the WebsitePreview client action's "new content" modal,
introduced in [1].
This commit adds a "redirect" parameter to the controller that will be
used by the 404 page, to keep the same behavior for this use case. But
when called from the webclient, it will return either the id of the
created view (for *.js, *.xml, ... urls), or the new page url, so that
the webclient is not fully reloaded when it is not necessary.
[1]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b
task-2687506
closesodoo/odoo#97408
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>