This migration is quite complex. Thorough testing is advised.
It also addresses part of #1891 (closed) by introducing a new SharingController which can be used in ViewModels and exposes LiveData. Some unused and legacy code was removed in the process.
It also fixes the unknown bug that the list scrolling position doesn't get restored when re-entering the discussion.
Closes #1870 (closed), #1873 (closed), #1881 (closed)