Implement additional schemas

[zone117x]

These to be determined based on developer feedback.

[friedger]

• Photos: https://forum.blockstack.org/t/proposal-create-a-photos-collection-for-cross-app-data-sharing/6863

• Documents: https://forum.blockstack.org/t/proposal-create-a-document-collection-for-cross-app-data-sharing/9286