[FIX] project: fix following on assignation

This commit fixes an issue since user_id was changed to user_ids.

The default behaviour of `_message_auto_subscribe_followers` could not
work with the user_ids field as it is only meant to work with user_id.

This also fixes a related issue due to the same problem where the
assignation emails were not sent.

TaskId-2691486

closes odoo/odoo#79831

X-original-commit: 3510251ad19d4394f4613924aa2af20abb7c35d9
Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>

addons/project/data/mail_template_data.xml

@@ -97,5 +97,18 @@
             <field name="lang">{{ object.rating_get_partner_id().lang }}</field>
             <field name="auto_delete" eval="True"/>
         </record>
+
+        <!-- You have been assigned email -->
+        <template id="project_message_user_assigned">
+<p style="margin: 0px;">
+    <span>Dear <t t-esc="assignee_name"/>,</span><br />
+    <span style="margin-top: 8px;">You have been assigned to the <t t-esc="model_description or 'document'"/> <t t-esc="object.display_name"/>.</span>
+</p>
+<p style="padding-top: 24px; padding-bottom: 16px;">
+    <a t-att-href="access_link" t-att-data-oe-model="object._name" t-att-data-oe-id="object.id" style="background-color:#875A7B; padding: 10px; text-decoration: none; color: #fff; border-radius: 5px;">
+            View <t t-esc="model_description or 'document'"/>
+    </a>
+</p>
+        </template>
     </data>
 </odoo>