DRY up variable references

By moving `var` delarations inside `notes.subscribe` handler.
Remove unnecessary braces around function bodies
Remove redundant `notes.subscribe`

We don't want to `merge` here because we're inside a `notes.subscribe`
handler already. Every time a new note comes, we'd be resubscribing to
`notes`. Just call `renderApp` and subscribe to `guesses` separately.
Refactor `notes.subscribe` handler
Merge branch 'refactor'
When correct, ignore guesses until next note

Prevents bugs caused by user clicking a button after guessing correctly
but before the next note was shown.

Rename `lastGuess` -> `guess` and `isLastGuessCorrect` ->
`isGuessCorrect` in this refactoring.