Create database migration

# Create Database Migration

## Instructions

1. Change directories into `ghost/core`: `cd ghost/core`
2. Create a new, empty migration file using slimer: `slimer migration <name-of-database-migration>`. IMPORTANT: do not create the migration file manually; always use slimer to create the initial empty migration file.
3. The above command will create a new directory in `ghost/core/core/server/data/migrations/versions` if needed, and create the empty migration file with the appropriate name.
4. Update the migration file with the changes you want to make in the database, following the existing patterns in the codebase. Where appropriate, prefer to use the utility functions in `ghost/core/core/server/data/migrations/utils/*`.
5. Update the schema definition file in `ghost/core/core/server/data/schema/schema.js`, and make sure it aligns with the latest changes from the migration.
6. Test the migration manually: `yarn knex-migrator migrate --v {version directory} --force`
7. If adding or dropping a table, update `ghost/core/core/server/data/exporter/table-lists.js` as appropriate.
8. Run the schema integrity test, and update the hash: `yarn test:single test/unit/server/data/schema/integrity.test.js`
9. Run unit tests in Ghost core, and iterate until they pass: `cd ghost/core && yarn test:unit`

## Examples
See [examples.md](examples.md) for example migrations.

## Rules
See [rules.md](rules.md) for rules that should always be followed when creating database migrations.