Django-webp-converter

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* Use the default_storage.save and default_storage.delete methods when saving
and deleting images.
* Add check for whether a WebP image already exists before attempting to save
the WebP image again following a cache miss.

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* Remove duplicate deletion of WebP folder in the `clear_webp_cache` command.

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* Switch to MIT license
* Fix bug where duplicate WebPImage models could be created due to the
non-uniqueness of nullable fields (https://github.com/tmiller02/django-webp-converter/issues/1).
* Change the default behaviour of the 'clear_webp_cache' command to
prompt for user confirmation before deleting the WebP image folder.
* Restructure project to split the webp_converter app out of the sample project
* Regenerate sample project using Django 3.0.