Add Flyway, run db migration in local PostgreSQL db

First step in my plan to switch away from MySQL.

database/postgresql/create-admin-user.sql

@@ -0,0 +1,3 @@
+CREATE USER cosmic_admin WITH ENCRYPTED PASSWORD 'redsnailshell';
+GRANT ALL PRIVILEGES ON DATABASE cosmic TO cosmic_admin;
+ALTER ROLE cosmic_admin WITH CREATEROLE;