Streamline old settings migration.

* Don't bother with FYAD and Gas Chamber settings, as we no longer have any way
  to determine whether the default light/dark theme was chosen.
* Delete the old keep sidebar open setting after migrating.
Stop updating table views in -viewWillDisappear: instead of -viewDidDisappear:.

Trying to track down some hard-to-reproduce crashes table view usage. This is
probably not going to fix anything but it feels more right.

Bonus: formatting.
Don't require taking address of KVOContext.

Error-prone.
Cover thread cell separator with action sheet on iPhone.