[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.