Configuration option to allow custom image sources

Since we allow customising the web vault's CSS, would it be a good idea to add a configuration option in the admin panel that lets us add a URL to the list of allowed img-src URLs in the content security policy? This is important for loading custom logos that may be hosted externally.

While we can override the content security policy with a reverse proxy like Traefik, this isn't ideal because Vaultwarden's content security policy can change across updates, and it's not possible to add a single URL; you must set the entire policy. I know the current settings list 'self' in img-src. Still, uploading custom logos to the machine running Vaultwarden isn't desirable for everyone, especially given the extra complexity of serving the logo file statically.

Happy to hear people's thoughts. I can write a PR once I've heard people's input.