[FIX] website_slides: fix choosing of templates in invite wizard

``channel_id`` field is used to compute ``render_model`` of invite wizard
used to filter templates to choose. It is a fake trigger ensuring the
render model field is computed, and then used in template domain
(see ``mail.composer.mixin`` and ``mail.render.mixin``).

As channel_id field is not available in view the domain computation for
template_id is not updated. No template is therefore available in template
m2o field in invite wizard. This commit fixes that by adding the channel_id
field in the view, meaning render_model is now computed.

Task-2709581

closes odoo/odoo#80947

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

addons/website_slides/wizard/slide_channel_invite_views.xml

@@ -8,6 +8,7 @@
                 <form string="Compose Email">
                     <group col="1">
                         <group col="2">
+                            <field name="channel_id" invisible="1"/>
                             <field name="partner_ids"
                                 widget="many2many_tags_email"
                                 placeholder="Add existing contacts..."