refs #6301
- changes createTable to use createTableIfNotExists, this is consistent with deletion
- splits out backup, reset, update and populate functions from migration/index into their own files
- moves the wrapped function for populatingDefaultSettings to fixtures.ensureDefaultSettings
- moves `modelOptions` down to the fixture files that actually use it
- adds test coverage for backup, reset and populate, but not for update as that needs refactoring
