feat: Reduce manual steps prior to running migration tool

[wbh1]

As reported in #32 and elsewhere, users often run into issues where they did not run an instance of Grafana connected to their destination Postgres database. This causes the migration to fail because Grafana did not create the tables that we're dumping data into.

Possible solutions:

• Shell script
• Docker compose
• Call Grafana's sqlstore migrator as a library to populate the tables ourselves
  • Complicated by the need to match migrations with what's in the existing DB which changes between Grafana versions and we can't dynamically import different versions of a library