Common's (broadcast) stream with each user

[elatif2020]

• Open for each user a dm stream of the user with Common system.
• Currently users should only be able to read this stream, not write new messages.
• Add system messages (should appear as such) there if the user is mentioned somewhere.
  It should be of the form: $userName mentioned you in $discussionName.
  If the discussion is the Home discussion of a space the displayed name should be the space name. $userName and $discussionName should be clickable.
• In the future we would like to add there also announcements from Common to users (those should appear as regular messages).