Merge pull request #81 from navikt/arena_data_created_at

Split into two statements to avoid setting created for existing rows

src/main/resources/db/migration/V16__arena_data_created.sql

@@ -0,0 +1,4 @@
+ALTER TABLE arena_data
+    ADD COLUMN created_at timestamptz DEFAULT NULL;
+ALTER TABLE arena_data
+    ALTER COLUMN created_at SET DEFAULT NOW();