Change generated avatar for empty name

The more I look at the blank white avatar that's generated when you haven't typed in a name yet, the more broken it looks.

This replaces it with a "👤" character on an orange background, chosen arbitrarily because I liked how it looked. There are any number of things that we could put in there, including any emoji; it doesn't really matter because the page won't allow you to proceed until you've typed a name, and then this avatar will be regenerated. So this is only about what the user sees when they first get to the Account page.

I think this is worth requesting a 3.34 UI freeze exception over, because to me the white circle just looks like some image failed to load.