Add backward compatible default to $theme-hero-image

[thisisdano]

Preview

Hero image theme setting default is now backward compatible: The $theme-hero-image theme variable now has a default equivalent to its effective value pre v2.2.0 so users that had a working version of their hero are more likely to see no issues upgrading to the latest version of USWDS.

• Although the new default includes a variable reference to $theme-image-path, users can change this to any path needed. It need not refer to $theme-image-path.
• This new default requires that the component theme variables (uswds-theme-components) be imported after uswds-theme-general. We suggest that uswds-theme-components be the last of the theme variables imported.

Old default:

$theme-hero-image: '..img/hero.png'

New default:

$theme-hero-image: '#{$theme-image-path}/hero.png'

Issue: #3118