Dividing fractions

As per https://trello.com/c/43oJvCNj/119-dividing-fractions-exercises

Exercise 0.5: Dividing a unit fraction by an integer
Exercise 1: Dividing an integer by a unit fraction
Exercise 2: Dividing a fraction by a fraction (should there be unit fraction by unit fraction? Maybe not that interesting)

Renamed word problems to word problems 2 (and tidied a bit).
Made a new word problems exercise based on word problems 2 but with only unit fractions.
Not sure how to pluralise words these days. I've used spans with `data-if`s in some places, but some words are in the form `$._("candy bars")` and I don't know what these are for or how to work with them.