Add new sizes for clothing

[jimthoburn]

Let’s add some new options for clothing sizes. The sizes are currently stored in the front-end code, so this should be just a matter of updating the UI.

• Baby boys
• Baby girls
• Boys
• Girls

During checkout, let’s show them in a <select>, like we’re already doing:
https://app.zeplin.io/project/5c186c18638b1baf04a579ac/screen/5c186d323e193baefe5ac155

This example shows how the sizes might be presented on the product detail page:
https://app.zeplin.io/project/5c186c18638b1baf04a579ac/screen/5cf5dbc7b117eb1d749aac7c

(We might want to wait for that page to be created before working on this issue.)