[api] add form signature route

[signebedi]

The idea here is that signatures on form submissions can be passed and handled using the rest API. They should not require data changes as these are just updates in the metadata, but should be given a journal entry. This allows us to reconstruct eg. that "User X signed form at TIMESTAMP".

Originally posted by @signebedi in #39 (comment)

[signebedi]

The underlying principle is that the user can only sign their own form. The question is what part of the application decides: the API, or the document database manager? The other question is how to decide whether a form can be signed. Is this set in the form config? Can all forms be signed? Is it a necessary part of the approval process? Do we set an onSign event hook?

[signebedi]

[form] add form event hooks
In past version of libreForms, we had event hooks for onCreate, onRead, onUpdate, onApproval, onDisapproval, and onDuplication. Perhaps we add onDelete and onSign event hooks here too.

[signebedi]

Add utils for managing signatures
In past versions we used Fernet, but I think that RSA will work better. The implication is that we will need to store user keypairs on the filesystem. This may or may not be a good thing...

[signebedi]

Form signatures I think should be based only on the data section of a given document, not the metadata. The reason is that the data, once signed, is expected not to change; the metadata is expected TO change... eg. as approvers sign a document to move it forward. In light of this, we don't want signature validation to fail just because a manager has approved a document.

[signebedi]

[api] Add a validate signature API route
The idea being that we should validate the signature on a document. No need to use the document database, this can all be done in the API. We should validate that the user has the permission to read the document, so we should call a read_document call to the document database, but again the heavy lifting is done in the API logic.

[signebedi]

[api] add a remove signature route
This route will remove a signature - valid or otherwise, from a form if the user has the permissions to do so (sign_own).