Fix EDB2 migrations

[zmousm]

• Make boolean, address, longitude/latitude fields (replaced in EDB2) nullable before removal
  • This is necessary so as to be able to perform a backwards migration, but it has an unfortunate implication: missing necessary data, it will not error as soon as the data migration happens but upon the nullable flag being reset
• Separate schema from UUID data backfilling in different migrations, so as to avoid a problem on databases that do support DDL transactions, such as PostgreSQL

Fix #73

[vladimir-mencl-eresearch]

Thanks for the quick fix @zmousm - tests all fine in my environment!