Time frame options for budgets and income (monthly, annual)

[eddyharrington]

Feature requests from CS50 Facebook group:

1. Budget scenario: as a user who budgets month to month and has varying income/expense levels as a result, I want the ability to configure my budgets monthly instead of annually, so that budgets can be scoped smaller and tracked more closely.

2. Income scenario: as a user with varying monthly income (e.g. freelancer), I want the ability to set income month to month instead of annually, so that weekly/monthly expenses can be tracked more clearly.