cf-forms: radio/checkboxes don't support checked disabled state

[anselmbradford]

Current behavior

• radio/checkbox inputs have a disabled state and selected state (checked), but only support a disabled state that isn't checked.

Expected behavior

• It's possible that a radio/checkbox input could be selected (checked), but also be disabled, yet the styles would break in this case, so the inputs should probably support that scenario.

Steps to replicate behavior

1. Run the docs and add checked="" to one of the checkbox/radio inputs and see it turn blue instead of gray.

[anselmbradford]

Fixed by https://github.com/cfpb/capital-framework/pull/955