Implementation of a universal upload allowing for multiple storage backends
starting with `database` and `s3`.
Allows clients to put and retrieve files from either client or server using
a blend of client and server actions.
Updates the signing endpoint to only apply changes to the Date field for the current signer. This is made possible through the addition of the `signedAt` column within the database.
Resolves the issue with one signer filling the date for all recipients and also ensures that the date of signing on a document won't always be todays date after each recipient has signed.
