FIX: set default on theme.key, if it has not yet been dropped (#6095)

db/migrate/20180716200103_add_theme_key_default.rb

@@ -0,0 +1,11 @@
+class AddThemeKeyDefault < ActiveRecord::Migration[5.2]
+  def up
+    if column_exists?(:themes, :key)
+      execute("ALTER TABLE themes ALTER COLUMN key SET DEFAULT 'deprecated'")
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end