[IMP] im_livechat, mail: Discuss UI in mobile

- Extract mobile specific code for the client action to a specific
  file
- Massive clean of less rules
- Unify channels preview of the systray dropdown and the mobile UI
  of Discuss
- Ensure to only load the last message of each channel in mobile,
  as this is not used in desktop (unless the user opens the systray
  menu)
- Make livechat available in Discuss mobile UI