Make organisation logo nullable

Now we have the org banner branding, not all organisations need a logo.
So it shouldn’t be an error to not provide one.

migrations/versions/0121_nullable_logos.py

@@ -0,0 +1,25 @@
+"""
+
+Revision ID: 0121_nullable_logos
+Revises: 0120_add_org_banner_branding
+Create Date: 2017-09-20 11:00:20.415523
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0121_nullable_logos'
+down_revision = '0120_add_org_banner_branding'
+
+
+def upgrade():
+    op.alter_column(
+        'organisation', 'logo',
+        existing_type=sa.VARCHAR(length=255),
+        nullable=True
+    )
+
+
+def downgrade():
+    pass