Move initial key backup to background
Since the initial key backup can take several minutes for some users, this moves
the upload step to the background. The create key backup flow now only marks
all sessions for backup synchronously, with the actual backup happening later.
The key backup panel in Settings gains a new row to show a summary of upload
status. Users are directed there if they wish to know if the backup is done.
The text in various related dialogs has also been tweaked to fit the new flow.
