Support displaying the appView without a valid quiz

(Useful in the case that the user requests specific words and we don’t
have any of them, so we can’t initialize a Quiz.)
Support constructing Quizes (and finding the associated verbs) from a list of infinitives
Don't show the nextPrompt on disabling a tense
Have Quiz throw an error when constructed with no verbs
Add support for more irregulars
Misc notes