Add Avatar component

[shawnbot]

This introduces an <Avatar> component with the following props:

• alt: string, passed through
• isChild: boolean, adds the avatar-child class if present
• size: number; default: 20, adds the avatar-small class if less than 24
• src: string

Examples:

<Avatar src='/primer.png' />
<Avatar size={40} src='/jonrohan.jpg' alt='jonrohan' />
<Avatar size={20} src='/shawnbot.jpg' isChild />

[shawnbot]

@jonrohan I think I addressed the unnecessary complexity of building the URL inside the component function by moving it into a getImageURL(username, params) function, which is now exported so that consumers can use it when setting src directly. See the tests for examples.

[jonrohan]

Do we need v=3 in the query string?

I think this is an image cache key that we use for our servers.

[shawnbot]

Note: I haven't yet implemented the parent-child component, but I was thinking of something like this doing it:

<Avatar username='github' size={48}>
  <Avatar username='primer' size={20} isChild />
</Avatar>

[shawnbot]

Let's hold off on this. 😁

[shawnbot]

Note: I needed the URLSearchParams ponyfill so that we can render the docs statically (i.e. outside of the browser). I'm honestly not sure how the test suite was running successfully without this. 🤔

[shawnbot]

We agreed that this is mixing concerns. The Avatar shouldn't be calculating URLs and should just take src directly. We might eventually need, for instance, a GitHubAvatar that does something like:

function GitHubAvatar({username = 'github', ...rest}) {
  return <Avatar {...rest} src={getUsernameURL(username)} />
}

[shawnbot]

We could limit this to a set of named sizes (small, medium, large), or have a way of mapping those to pixel sizes.

[shawnbot]

TODO: pass alt through as-is.

[shawnbot]

I've implemented this in our examples so that we can easily change avatars and sizes.

[shawnbot]

This is so much simpler now. 🎉

[shawnbot]

Note that I've sorted these imports alphabetically.

[shawnbot]

Okay, this is ready for a final review.

[jonrohan]

Super clear 💯