Add bundle logo

[brisvag]

On suggestion from @Czaki, a logo to use to refer to the napari bundle.

[brisvag]

note: take gradient from #25 if that's merged

[jni]

I'm +/-1 on this one. I feel like the ribbon should have more texture, and anyway since the bundle should use the normal app logo, there's limited uses for this. I'll let others decide.

[brisvag]

IMO if this sees any use is only for clarity in the docs when referring to the bundle, or for communications, but yeah otherwise I see little use.

[TimMonko]

I think I'm -1 on this. Mainly because it would be confusing to use this to refer to the bundle in docs and such, if when someone uses the bundle, its not the logo used. I also think it looks more like a Christmas-type logo than a bundle one. I hate to be a downer though, so I do just want to say I appreciate the time and effort you put in!

[brisvag]

No worries, this was pretty quick to do actually :P Let's put on hold for now then, let's see what others say, but for now it sounds like we're more on the -1 than +1.

changing to draft to prevent accidental merge before this is updated with whatever gradient we end up with at #25.