Make an array to house the word label outlets

This simplifies the assignment of the individual passphrase words
to each label's text property.
Simplify tappedLabel

At this time there is no need to check each label outlet to
determine which one was tapped. Simply checking if the view
controller's view was not the tapped view is sufficient.

If more views are added, this should be reconsidered.
Toggle selected passphrase label's highlighted state

This changes the color of selected passphrase words to red, then
returns them to their default color when unselected.
Provide ability to replace selected words
Clear selected words on passphrase generation