Add Northgard (game) icon

[aaheen]

These should work, commit names & messages should be self-explanatory. They're a bit more detailed than my previous submission of the proton-mail icon, but I think they render well, especially on HiDPI displays. Let me know if I should make any changes.

Edit: Forgot to mention that you should probably squash this, as the two separate commits are just from me forgetting to add the symlink before the first commit.

[aaheen]

Clicked a few buttons on my end by mistake. Closing this, tweaking it, and re-opening it for the sake of simplicity.