For the 'big picture' make categories configurable

[davidschlachter]

Currently, the 'big picture' page is hard-coded to consider some categories as income, and to ignore others. This should be configurable with some sort of config file. Logistically, this would mean that we fetch the sum of each category over the period (including 'no category'), then sum all these according to the rules in the config.

Configuration could maybe be stored in the database. All this stuff should have better unit tests (they're kind of sketchy now).