Add test site banner and disable submission

[hartsick]

To support autocomplete demo application.

• Text only in English, so put directly into HTML
• Demo banner and disabling always present, since I assume we're just a demo application at this point and wanted to get up quickly.
• Disables submission part of feature tests
• Adds in background color class from USWDS we need to match Login demo banner.

Note in case this impacts anyone else: I had to totally delete the compiled files in the public directory to get CSS and JS changes locally, even after turning off dev cache and running yarn build and yarn build:css.

Closes #315

[rahearn]

Note in case this impacts anyone else: I had to totally delete the compiled files in the public directory to get CSS and JS changes locally, even after turning off dev cache and running yarn build and yarn build:css

FWIW, for me the banner rendered properly after running rake assets:clobber before bin/dev