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.
On larger documents we could accidentally start trying to render fields
while not all pages of the PDF have loaded due to us checking for a
single page existing. This would cause an error to be thrown, hard
locking those documents.
This change resolves this by grabbing the highest page number from the
given fields and using it for the visibility check instead.
