Add columns for on whose behalf agreement signed

This is changing because we’re going to introduce accepting contracts
and MoUs online.

Previously
---
We had one column for who signed the agreement, which is foreign keyed
to the user table. This is still relevant, because there will always be
a user who is clicking the button.

Now
---
We add two new fields for the name and email address of the person on
whose behalf the agreement is being accepted. This person:
- is different from the one signing the agreement
- won’t necessarily have a Notify account

app/models.py

@@ -344,6 +344,8 @@ class Organisation(db.Model):
         db.ForeignKey('users.id'),
         nullable=True,
     )
+    agreement_signed_on_behalf_of_name = db.Column(db.String(255), nullable=True)
+    agreement_signed_on_behalf_of_email_address = db.Column(db.String(255), nullable=True)
     agreement_signed_version = db.Column(db.Float, nullable=True)
     crown = db.Column(db.Boolean, nullable=True)
     organisation_type = db.Column(db.String(255), nullable=True)
@@ -386,6 +388,8 @@ class Organisation(db.Model):
             "agreement_signed": self.agreement_signed,
             "agreement_signed_at": self.agreement_signed_at,
             "agreement_signed_by_id": self.agreement_signed_by_id,
+            "agreement_signed_on_behalf_of_name": self.agreement_signed_on_behalf_of_name,
+            "agreement_signed_on_behalf_of_email_address": self.agreement_signed_on_behalf_of_email_address,
             "agreement_signed_version": self.agreement_signed_version,
             "domains": [
                 domain.domain for domain in self.domains