Expand NSF theme settings to include our custom footer menus

[fureigh]

User story

As a site administrator, I want the UI to reflect the state of the code so that I don't have to take nonobvious paths to modify things.

Motivation

The USWDS theme includes a 'Bypass USWDS menu processing for these menus' option, with checkboxes for each standard menu. Those checkboxes don't yet include the two custom menus (footer-national-science-fou and footer-research-gov) we're using in the footer region.

Acceptance criteria

• When a site developer/administrator goes to /admin/appearance/settings/nsf, they see checkboxes for Footer: National Science Foundation and Footer: Research.gov after the checkbox for Footer menu.
• After that user clicks one of those checkboxes and then clicks the Save configuration button, the footer menus continue to display.
• After that user clicks one of those checkboxes and then clicks the Save configuration button and clears the cache, the footer menus' markup no longer includes USWDS alterations (usa-*, etc.).

Technical notes

Implementing this will require the use of nsf/theme-settings.php (copied from the USWDS theme and modified to override) and the Form API, specifically hook_form_system_theme_settings_alter().

[fureigh]

Also use hook_theme_suggestions_block_alter() (a la uswds_theme_suggestions_block_alter() in /uswds/includes/menu.inc)).