Generative avatar/icon design for accounts and tokens

@aulneau to chime in here on technical constraints.

For a user an avatar or icon is helpful in differentiating between accounts and tokens. Each of these will have a unique address, and we can use these addresses to generate a visual that is unique as well. We can use shapes, colors (+gradients) and type. Keep in mind that these will generally be pretty small, around 36x36 px.

I'd like to propose the following guiding principles:

- They should be easy to differentiate
- They should be easy to remember
- They should be on brand

We currently use gradients for this, its colors constrained by a HSL range, with an optional letter that represents the first letter of the token.