Add support for profile images based on GitHub theme preferences

[tenthirtyam]

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Today, GitHub-flavored markdown supports light-

![Dark](https://example.org/image-dark.png#gh-dark-mode-only)
![Light](https://example.org/image-light.png#gh-light-mode-only)

This is a nice feature... but what about using the concept for the Profile page?

A nice feature would be to support profile images based on the user's GitHub theme preferences, but also select one as the default.

Ryan Johnson
[at]tenthirtyam