Clears the undo manager when resigning first responder. Also, fixes the undo manager displaying undo/redo options when there was nothing to undo/redo.
Clears the undo manager when resigning first responder. Also, fixes the undo manager displaying undo/redo options when there was nothing to undo/redo.

slackhq/SlackTextViewController,slackhq/SlackTextViewController,kobim/SlackTextViewController,kobim/SlackTextViewController