Feature/sc 27523/sheets green create button

This is a first pass (and thus a draft pull request) of the generic Button component. It assumes children is text and can optionally display an image. Example usages:

const CreateSheetsButton = () => {
  const img = <img src="/static/icons/new-sheet-black.svg" alt="make a sheet icon" id="sheetsButton"/>;
  return <Button img={img} classes={{small: 1}} href="/sheets/new">Create</Button>
}

and

<Button classes={{getStarted: 1}} href="/sheets/393695">Get Started</Button>

Sefaria / Sefaria-Project

Feature/sc 27523/sheets green create button #1914