Don't add CREATE SCHEMA public commands to down migrations

[gregtyler]

Purpose

Down migrations generated by Doctrine include a CREATE SCHEMA public command. This is erroneous and causes the down migrations to fail (because the public schema already exists).

This is a known issue in Doctrine DBAL.

Approach

I removed the CREATE SCHEMA public command from existing migrations, and also introduced a listener which stops them from being added unless necessary (e.g. if we start using multiple schemas).

Learning

I took the fix from a comment on the GitHub issue. This seems to be the accepted solution for now, awaiting Doctrine to fix the issue permanently.

Checklist

• I have performed a self-review of my own code
• [N/A] I have updated documentation (Confluence/GitHub wiki) where relevant
• [N/A] I have added tests to prove my work, and they follow our best practices
• [N/A] I have successfully built my branch to a feature environment
• New and existing unit tests pass locally with my changes (docker-compose run --rm api sh scripts/apiunittest.sh)
• There are no Composer security issues (docker-compose run api php app/console security:check)
  • False positive on Symfony error fixed in DDPB-2617: Create a test account for a lay deputy with a co deputy #549
• [N/A] The product team have tested these changes