Change Item.value to be an integer, in cents, make other values more explicit

[armahillo]

We use dollar values in a few places.

These need to be integers, in cents, so that we don't have floating point issues.

We should also rename existing value fields to have _cents in the name so it's explicit what kind of values we're expecting there

Criteria for Completion

• Change Item.value to an integer
• Change Item.value to Item.value_in_cents
• Change Purchase.amount_spent to Purchase.amount_spent_in_cents

[jeduardo824]

I'm working on this

[jeduardo824]

Should I change the label "Value" to "Value in cents" on the views?