Allow setting of a custom logo

[dsluijk]

There are two main approaches to this, we should look into which one is desirable.

1. Use the admin panel to upload an image. Adds major complexity, but a better UX.
2. Specify the image in environmental variables, perhaps with a volume mount. Worse UX, simpler to set up.

Either way we should support color mode, so we should allow the user to specific two different images.