Replace custom validate_file_extension with built-in FileExtensionValidator

Ghost User
2019-03-27T20:30:47+01:00

### One-sentence description Replace custom `validate_file_extension` with built-in [`FileExtensionValidator`](https://docs.djangoproject.com/en/2.1/ref/validators/#fileextensionvalidator). ### Why? Less custom code to maintain.