Add filename to dvla_organisation table

Added a filename column to the dvla_organisation table and populated it
with the filenames that are currently hard-coded in template-preview.
The filenames for letter logos are going to be stored in the database,
instead of in template-preview.

migrations/versions/0237_add_filename_to_dvla_org.py

@@ -0,0 +1,56 @@
+"""
+
+Revision ID: 0237_add_filename_to_dvla_org
+Revises: 0236_another_letter_org
+Create Date: 2018-09-28 15:39:21.115358
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.sql import text
+
+
+revision = '0237_add_filename_to_dvla_org'
+down_revision = '0236_another_letter_org'
+
+
+LOGOS = {
+    '001': 'hm-government',
+    '002': 'opg',
+    '003': 'dwp',
+    '004': 'geo',
+    '005': 'ch',
+    '006': 'dwp-welsh',
+    '007': 'dept-for-communities',
+    '008': 'mmo',
+    '009': 'hmpo',
+    '500': 'hm-land-registry',
+    '501': 'ea',
+    '502': 'wra',
+    '503': 'eryc',
+    '504': 'rother',
+    '505': 'cadw',
+    '506': 'twfrs',
+    '507': 'thames-valley-police',
+    '508': 'ofgem',
+    '509': 'hackney',
+    '510': 'pension-wise',
+    '511': 'nhs',
+    '512': 'vale-of-glamorgan',
+    '513': 'wdc',
+    '514': 'brighton-hove',
+}
+
+
+def upgrade():
+    conn = op.get_bind()
+    op.add_column('dvla_organisation', sa.Column('filename', sa.String(length=255), nullable=True))
+
+    for org_id, org_filename in LOGOS.items():
+        conn.execute(text("""
+            UPDATE dvla_organisation SET filename = :filename WHERE id = :id
+        """), filename=org_filename, id=org_id)
+
+
+def downgrade():
+    op.drop_column('dvla_organisation', 'filename')