Align multiplication exercises to common core

See https://trello.com/card/multiplication-exercises/5166dc745b4dc7926c000a71/113

Test plan: local

Auditors: alpert
Align division exercises to common core

See https://trello.com/card/division-exercises/5166dc745b4dc7926c000a71/114

Test plan: local

Auditors: alpert
Don't validate empty number answers

This only seems to come up in a multiple with, say, 2 text boxes. 1 is required, the other has fallback. The required one gets validated and was returning false until this change.

Test plan: Open division_2 and hit enter

Auditors: alpert
Align adding decimals exercises to common core

See https://trello.com/card/adding-decimals-exercises/5166dc745b4dc7926c000a71/115

Test plan: local

Auditors: alpert
Align subtracting decimals exercises to common core

See https://trello.com/card/subtracting-decimals-exercises/5166dc745b4dc7926c000a71/116

Test plan: local

Auditors: alpert
Align multiplying decimals exercises to common core

See https://trello.com/card/multiplying-decimals-exercises/5166dc745b4dc7926c000a71/117

Test plan: local

Auditors: alpert