Feature: Service ping, redesign docker status

[shamoon]

As discussed, this (draft for now) PR implements the ping which uses a HEAD request, also shows estimated latency if status is 200. Changes look of docker status too. As noted users would need to be careful about which URL to ping, if the public domain is behind a reverse-proxy this will potentially falsely always return status 200, instead local URL / ip could be specified to test http status.

Closes #248, closes #78

services.yaml

    - Paperless-ngx:
        href: https://paperless.domain.com
        description: My awesome Paperless-ngx instance
        icon: paperless.png
        ping: https://google.com
        server: docker
        container: paperless