Make fid nullable

I'm not totally sure what we do with sources with no FID, but we are
getting it from OFXHome so I'm going to keep it

alembic/versions/3ec1289eae7a_ofxaccount_fid_nullable.py

@@ -0,0 +1,28 @@
+"""ofxaccount_fid_nullable
+
+Revision ID: 3ec1289eae7a
+Revises: 3bac184298c9
+Create Date: 2016-07-20 15:36:28.157723
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '3ec1289eae7a'
+down_revision = '3bac184298c9'
+branch_labels = None
+depends_on = None
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+def upgrade():
+    op.alter_column('ofxsource', 'fid',
+               existing_type=sa.VARCHAR(length=255),
+               nullable=True)
+
+
+def downgrade():
+    op.alter_column('ofxsource', 'fid',
+               existing_type=sa.VARCHAR(length=255),
+               nullable=False)