[IMP] http_routing, portal, website_*: clean 404 templates

A nicer 404 layout was introduced with e9106f8f but the specs got changed
just after it was merged.

It has been decided to make the 404 fully editable (before, everything was
fully editable except the popular page div).

In order to do this, the 404 template can't have inherited views, which brings
the following changes:
1. Remove every main website module xpath view adding their most popular page
2. Remove the xpath view in portal to add popular page part (was not needed
   in http_routing/web). It has been decided that having `Home` ('/' url) even
   without portal and/or website is not a big deal.

Those changes allow the 404 template to be written in a single view without any
inherited views.
The 404 will be the same for backend only databases, portal and website.

task-1966460

closes odoo/odoo#40637

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>