Only generate companion CSP one time to reuse it

[Fijxu]

Closes #5496

CSP with this in config.yml:

invidious_companion:
  - private_url: "http://localhost:8282/companion"
    public_url: "http://localhost:8282/companion"
  - private_url: "http://localhost:8282/companion"
    public_url: "http://localhost:8282/companion"
  - private_url: "http://localhost:8282/companion"
    public_url: "http://localhost:8282/companion"

Looks like this:

Content-Security-Policy: default-src 'none'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; font-src 'self' data:; connect-src 'self' http://localhost:8282 http://localhost:8282 http://localhost:8282; manifest-src 'self'; media-src 'self' blob: http://localhost:8282 http://localhost:8282 http://localhost:8282; child-src 'self' blob:; frame-src 'self'; frame-ancestors 'none'