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.
Implements a bulk send feature allowing users to upload a CSV file to
create multiple documents from a template. Includes CSV template
generation, background processing, and email notifications.
