Add new endpoint to generate SVG badges

In order for users to add Software Heritage SVG badges in README files to indicate their code is archived, a new endpoint should be added to the web application to generate them.

This endpoint could have the following url: /badge/(object_type)/(object_id)/ where object_type will be one of content, directory, origin, release, revision, snapshot and object_id the associated identifier (either an url or a sha1).

That new endpoint should also adds Cross-Origin Resource Sharing (CORS) headers to responses in order for the badges to be accessed on other domains (for instance from https://github.com). Hopefully, we can use django-cors-headers to easily do that.

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Migrated from T2090 (view on Phabricator)