Add support for custom content disposition and respect WAGTAILDOCS_SERVE_METHOD for public files

[zerolab]

At the moment public files are redirected to their public URL and rely on whatever storage is used to provide content disposition

Wagtail 2.11+ allows customising content-disposition: https://github.com/wagtail/wagtail/blob/stable/2.11.x/wagtail/documents/models.py#L181

Would be good to let the core serve method complete its thing for public files, rather than plain redirect to the file