[IMP] mail: add moderation on channels

Purpose of this commit is to allow moderation on incoming messages in
discussion channels. On some channels on which moderation is required
messages should be in a pending moderation stage. Moderators can accept
or refuse messages as well as always allow or ban messages coming from
a given set of emails.

Channels now have an option to be moderated. Moderators can be added on
channels. They have access to a specific UI in Discuss to see and take
action on messages waiting for moderation.

Concerning mail.thread message that are pending moderation are not notified.
It means nobody receives a notification about them. Moderation process calls
the notification once the message is validated.

Various features included in this commit :

 * a model is added to store the decision about emails, allow or ban;
 * access rights are updated so that only moderators can modify moderation
   fields on message;
 * specific bus notifications are send to moderated people as well as to
   moderators on incoming emails as well as when a decision is taken;
 * options are added on channels to send explanations to moderated emails;
 * options are added on channels to write and send guidelines explaining
   why and how moderation is performed;
 * a reminder is send daily to moderators with remaining messages to moderate;
 * discuss UI is adapted and a new channel is added below Inbox and Starred
   giving access to moderation tools;
 * chanenl UI is adapted allowing to moderate directly inside channels;

This commit is linked to task ID 29521. Closes #21921.

addons/mail/data/mail_data.xml

@@ -226,5 +226,37 @@
 </td></tr>
 </table>
         </template>
+
+        <!-- Channel and moderation related data -->
+        <template id="mail_channel_notify_moderation">
+<div>
+    <p>Hello <t t-esc='record.name'/></p>
+    <p>You have messages to moderate, please go for the proceedings.</p><br/><br/>
+    <div style="text-align: center;">
+        <a href="/web#action=mail.mail_channel_action_client_chat&amp;active_id=channel_moderation" style="background-color: #1abc9c; padding: 20px; text-decoration: none; color: #fff; border-radius: 5px; font-size: 16px;" class="o_default_snippet_text">Moderate Messages</a>
+        <br/><br/><br/>
+    </div>
+    <p>Thank you!</p>
+</div>
+        </template>
+
+        <record id="mail_template_channel_send_guidelines" model="mail.template">
+            <field name="name">Partner: Send channel guidelines</field>
+            <field name="model_id" ref="base.model_res_partner"/>
+            <field name="email_from">${object.company_id.catchall or object.company_id.email|safe}</field>
+            <field name="partner_to">${object.id}</field>
+            <field name="subject">Guidelines of channel ${ctx['channel'].name}</field>
+            <field name="body_html" type="xml">
+<div>
+<p>Hello ${object.name or ''},</p>
+<p>Please find below the guidelines of the ${ctx['channel'].name} channel.</p>
+<p>
+${ctx['channel'].moderation_guidelines_msg}
+</p>
+<p></p>
+</div>
+</field>
+            <field name="user_signature" eval="False"/>
+        </record>
     </data>
 </odoo>