[IMP] website_theme_install: add customize_show on theme.ir.ui.view

Before this commit, having a `customize_show` on an ir.ui.view record in a
theme module would be ignored when creating the theme.ir.ui.view as the field
would not exists.
Actually it would log a warning on create (module install) and raise a
traceback on module update (theme update).

closes odoo/odoo#39216

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