[IMP] website_event_[exhibitor|meet|track]: add link for no content submenu buttons

This commit modifies texts proposing to the users to create new
exhibitor/meeting_room/tracks by adding links redirecting the user to the
backend allowing them to easily create new records without having to go to
the backend again.

It also prevents the user from being redirected to the main event page when
toggling the allow room creation submenu, instead it will only reload the
current page, thus preventing the user from losing the community page.

Task-2531104

closes odoo/odoo#71825

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>