Make definitions consistent

[annekainicUSDS]

Description

Turn all schema and uiSchema in definitions into functions to make their usage consistent. Here's an example of what that usage would look like: https://github.com/usds/us-forms-system-starter-app/compare/test-new-schema-additions-to-usfs?expand=1.

Connected to #222

[dmethvin-gov]

LGTM. We can treat the need for .schema({ more_stuff }) as YAGNI until proven otherwise.

[annekainicUSDS]

Sounds good! Will merge now and we can always edit/add more later.

[bernars-usa]

Are there any documentation updates to make based on these changes?

[dmethvin-gov]

Yes indeedy! We'll want to rewrite common-definitions.md to explain the new way of using the definitions since they return both schema and uiSchema. The good news is that they're all consistently functions now and we'll just need to explain what the arguments are but nothing else.