[ticket/14434] Allow non-migration files inside migrations folder (continued)
* VSEphpbb/ticket/14434:
[ticket/14434] Refactored to check migrations when setting them
[ticket/14434] Check migrations in the database updater task
[ticket/14434] Do not include non-migrations in CLI list
[ticket/14434] Remove redundant conditional
[ticket/14434] Fix whitespace mistakes
[ticket/14434] Remove recursion to simplify is_migration method
[ticket/14434] Extract migration check to a reusable method
[ticket/14434] Schema generator should ignore migration helpers