As mentioned in the previous commit, `<Main>` doesn't really
belong in App.js, it instead belongs in LandingPage.js, because
it creates the html/css wrapper for the main contents, as a
sibling to the `<PageHeader>` component used in LandingPage.js.
However, including `<Main>` in LandingPage.js will cause the
tests to fail. Therefore, just the html and classes that
`<Main>` creates is used in LandingPage.js.
