Drop checksum from ImageStorage table

The checksum field was removed from the ImageStorage model in #815,
but was never dropped from the database.  This adds a migration to
drop the unused column.

data/migrations/versions/c91c564aad34_drop_checksum_on_imagestorage.py

@@ -0,0 +1,22 @@
+"""Drop checksum on ImageStorage
+
+Revision ID: c91c564aad34
+Revises: 152bb29a1bb3
+Create Date: 2018-02-21 12:17:52.405644
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'c91c564aad34'
+down_revision = '152bb29a1bb3'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade(tables):
+    op.drop_column('imagestorage', 'checksum')
+
+
+def downgrade(tables):
+    op.add_column('imagestorage', sa.Column('checksum', sa.String(length=255), nullable=True))