Adds support for multipart/form-data requests in the public api
allowing documents to be uploaded without having to perform a secondary
request.
Need to rollout further endpoints for envelopes and templates.
Need to change how we store files to not use `putFileServerSide`
This PR is handles the changes required to support envelopes. The new
envelope editor/signing page will be hidden during release.
The core changes here is to migrate the documents and templates model to
a centralized envelopes model.
Even though Documents and Templates are removed, from the user
perspective they will still exist as we remap envelopes to documents and
templates.
