[FIX] mail: configure members of channels

Not being able to add / remove members of a channel through its form view is
quite limitating.

Also added the followers widget in the channel form view in technical mode.
Indeed it is sometimes necessary to be able to check a channel followers.

addons/mail/views/mail_channel_views.xml

@@ -136,7 +136,7 @@
                                 </group>
                             </page>
                             <page string="Members">
-                                <field name="channel_partner_ids" mode="tree" readonly="1">
+                                <field name="channel_partner_ids" mode="tree">
                                     <tree string="members">
                                         <field name="name"/>
                                         <field name="email"/>
@@ -145,6 +145,9 @@
                                 </field>
                             </page>
                         </notebook>
+                        <div class="oe_chatter" groups="base.group_no_one">
+                            <field name="message_follower_ids" widget="mail_followers"/>
+                        </div>
                     </sheet>
                 </form>
             </field>