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

migrations/versions/0296_agreement_signed_by_person.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0296_agreement_signed_by_person
+Revises: 0295_api_key_constraint
+Create Date: 2019-06-13 16:40:32.982607
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '0296_agreement_signed_by_person'
+down_revision = '0295_api_key_constraint'
+
+
+def upgrade():
+    op.add_column('organisation', sa.Column('agreement_signed_on_behalf_of_email_address', sa.String(length=255), nullable=True))
+    op.add_column('organisation', sa.Column('agreement_signed_on_behalf_of_name', sa.String(length=255), nullable=True))
+
+
+def downgrade():
+    op.drop_column('organisation', 'agreement_signed_on_behalf_of_name')
+    op.drop_column('organisation', 'agreement_signed_on_behalf_of_email_address')