[FEATURE] Light/dark mode images

[jdabtieu]

Is your feature request related to a problem? Please describe.

Sometimes images should swap from light to dark or vice versa depending on the color theme the user has. Admins should be able to be able to specify an image to use in light mode, and also in dark mode.

Describe the solution you'd like

Specify an alternative image for light/dark mode, like what DMOJ has