Add dbmigrate scrip, other misc work

data/sql/dbmigrate-26_to_27.sql

@@ -0,0 +1,10 @@
+-- Script to migate from db version 6 to 27
+-- Nothing to do
+
+CREATE TABLE IF NOT EXISTS collection_attributes (
+    id INTEGER REFERENCES source(id) ON DELETE CASCADE ON UPDATE CASCADE DEFERRABLE INITIALLY DEFERRED, -- source id, null for local source
+    k TEXT NOT NULL,
+    v TEXT NOT NULL
+);
+UPDATE settings SET v = '27' WHERE k == 'schema_version';
+