fix conductor backstack when chat notification is opened
when opening a chat notification, the old chat should not be kept in the backstack. so when clicking the back button when coming from a chat that was opened by a notification, now the ConversationList opens.
by the way, this also avoids to run into bug #2181 (but it's root cause is not solved yet)
Signed-off-by: Marcel Hibbe <dev@mhibbe.de>