Add dark_mode_url to webpage entity

[maurerle]

#7304 gave me inspiration to suggest a dark_mode_url for other entities like webpages too.

I am using grafana to visualize some embeded panels.
The URL to grafana looks like: http://grafana-host/d-solo/XxXx-asZk/dashboard?orgId=1&theme=light&panelId=1

This does not look good on dark ui (mobile phone).
I'd like to provide a second url with theme=dark.
The easiest and most generic way to do so would probably be to provide an optional dark_mode_url in the webpage entity