feat(Avatar): Add initials

[ashwoodall]

This PR adds the ability for a name prop to be passed to Avatar and creates initials if an image src is not defined. It handles passed first + last name, first (single) name, initials (2 chars), or no name prop.

[martahj]

This is looking good!

Since we're temporarily unable to use Now, could you please do the following:

• Pull in the latest changes
• Add a working example of this being used to a demo page
• Run yarn dev
• Verbally verify that this is behaving as expected

[ashwoodall]

@martahj This is our beautiful Avatars demo page now.

[martahj]

Such a beautiful demo page. Look at that gradient behind 'hi'.