This commit introduces a nicer 404 page, which is basically the same layout as
the one used on Odoo.com.
Also, the 404 is now fully customizable, blocks can be drag'd & drop'd.
task-1966460
closesodoo/odoo#38901
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>
