add CSP header to files in `/media`
This fixes a security issue of stored cross-site scripting via embedding
JavaScript in SVG files by a malicious user with `can_save_media`
capability.
This can be exploited if:
- the uploaded file is served from the same origin as authentik, and
- the user opens the uploaded file directly in their browser
Co-authored-by: Jens L. <jens@goauthentik.io>