Fix thumbnail command (#3243)

- New image size "preview" was not being generated
- Check was looking for existance of "thumbnail" (which did exist)
- Updated so that all image sizes are generated on a migration

InvenTree/InvenTree/management/commands/rebuild_thumbnails.py

@@ -4,9 +4,7 @@
 """
 
 import logging
-import os
 
-from django.conf import settings
 from django.core.management.base import BaseCommand
 from django.db.utils import OperationalError, ProgrammingError
 
@@ -27,10 +25,7 @@ class Command(BaseCommand):
             return
 
         img = model.image
-        url = img.thumbnail.name
-        loc = os.path.join(settings.MEDIA_ROOT, url)
 
-        if not os.path.exists(loc):
         logger.info(f"Generating thumbnail image for '{img}'")
 
         try: