[refactor] Attachment images (#11961)

* Add new Attachment model fields:

- is_image
- thumbnail

* Cache if the attachment is an image

* Add new setting for controlling max upload size

* Validate uploaded attachment file

* Add tqdm for progress bars

* Refactor migrations

- Don't need is_image field
- Can introspect from the thumbnail

* Data migration for existing attachments

* Bump API version

* Update tests and validators

* Add "is_image" field to the Attachment model

* Offload to background task

* Implement unit tests

* Docs

* Add unit test for data migration

* Additional unit test

* Omit migration tests from code coverage

* Additional unit tests

codecov.yml

@@ -1,3 +1,6 @@
+ignore:
+  - "src/backend/InvenTree/**/test_migrations.py"
+
 coverage:
   status:
     project: