Allow the homepage text block to use an image background

[apburnes]

When a user is managing the homepage, allow a user to add an image background to a text block section

Notes

• See the homepage hero element for setting the background

Acceptance Criteria

• Allow an optional background image for the text block
• Update the pages-site-gantry to render a bacground image

[kevinsmaster5]

Editor branch created
main...feat-add-image-background-homepage-text-block

Gantry PR
cloud-gov/pages-site-gantry#182