Some notification observers are not removed

[crazytonyli]

There are many places that calls NotificationCenter.addObserver(forName:object:queue:using:) and discards its return value, which leads to the observer not being unregistered.

Observers not getting unregistered will cause some problems:

1. The block argument will not be released, which causes some objects not being released in some cases.
2. The observer code is executed at a time that you don't expect them to execute.

[peril-wordpress-mobile]

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