Add proper nin favicon, replacing yeoman

[stianjensen]

This marks a big milestone in nin's history, removing the last piece of the once-yeoman-scaffolded Angular 1.x application, the yeoman mascot!

The center-square-cropped nin logo takes its place and will hopefully serve nin equally good in the future.

[sigvef]

Perhaps we should use the non-cropped version of the logo?

[sigvef]

Also, RIP

[stianjensen]

I took the extra minute of effort and made a non-cropped version now 😅