Allow to set separate logo for dark mode

[salim-b]

Describe the feature request

When using a black-and-white or dark logo with transparent background[*], it becomes barely visible when dark mode is toggled on. The same is true for light mode with a white/light logo with transparent background. Thus it would be nice if we could configure two seperate logo files/URLs, one for the (default) light mode and one for the dark mode.

Config could become ui.logo.light and ui.logo.dark, maybe with a fallback to the current behaviour when ui.logo is directly set to a string instead of the two subkeys.

Why do you personally want this feature to be implemented?

It would improve Gatus' web frontend aesthetically. 💅

How long have you been using this project?

3 days (awesome project! ❤️)

Additional information

[*] Example Gatus instance with such a logo is found here: https://status.votelog.ch/