Common's (broadcast) stream with each user

- [ ] 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).