make job.created_by nullable

Since letter jobs from the API aren't created by any single individual,
lets make created_by nullable. Note: We'll have to make sure that we
update the admin app to handle these jobs nicely

migrations/versions/0113_job_created_by_nullable.py

@@ -0,0 +1,24 @@
+"""empty message
+
+Revision ID: 0113_job_created_by_nullable
+Revises: 0112_add_start_end_dates
+Create Date: 2017-07-27 11:12:34.938086
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0113_job_created_by_nullable'
+down_revision = '0112_add_start_end_dates'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+def upgrade():
+    op.alter_column('job', 'created_by', nullable=True)
+
+
+def downgrade():
+    # This will error if there are any jobs with no created_by - we'll have to decide how to handle those as and when
+    # we downgrade
+    op.alter_column('job', 'created_by', nullable=False)