[BB2-1305] Add US Banner gov site top css resources

[JFU-GIT]

JIRA Ticket:
BB2-1305

User Story or Bug Summary:

This PR added USABanner sccs (pulled from CMS design system: https://github.com/CMSgov/design-system) to BB2 bluebutton-css, which is needed to build css files for BB2 SBX web pages and BB2 site static web pages.

What Does This PR Do?

1. Added US Banner sccs resource to bluebutton-css
2. Modified gulp files to generate BB2 css files
3. Saved css fragments from legacy BB2 css for some web widgets where legacy look & feel are preferred

What Should Reviewers Watch For?

1. the BB2 css can be generated as per readme
2. the legacy-override.css file presents and can be consumed as expected by BB2 SBX web pages and BB2 site static web pages

If you're reviewing this PR, please check these things, in particular:

• TODO

What Security Implications Does This PR Have?

Submitters should complete the following questionnaire:

• If the answer to any of the questions below is Yes, then here's a link to the associated Security Impact Assessment (SIA), security checklist, or other similar document in Confluence: N/A.
  • Does this PR add any new software dependencies? No.
  • Does this PR modify or invalidate any of our security controls? No.
  • Does this PR store or transmit data that was not stored or transmitted before? No.
• If the answer to any of the questions below is Yes, then please add @StewGoin as a reviewer, and note that this PR should not be merged unless/until he also approves it.
  • Do you think this PR requires additional review of its security implications for other reasons? No.

What Needs to Be Merged and Deployed Before this PR?

This PR cannot be either merged or deployed until the following pre-requisite changes have been fully deployed:

• CMSgov/some_repo#42

Submitter Checklist

I have gone through and verified that...:

• This PR is reasonably limited in scope, to help ensure that:
  1. It doesn't unnecessarily tie a bunch of disparate features, fixes, refactorings, etc. together.
  2. There isn't too much of a burden on reviewers.
  3. Any problems it causes have a small "blast radius".
  4. It'll be easier to rollback if that becomes necessary.
• I have named this PR and its branch such that they'll be automatically be linked to the (most) relevant Jira issue, per: https://confluence.atlassian.com/adminjiracloud/integrating-with-development-tools-776636216.html.
• This PR includes any required documentation changes, including README updates and changelog / release notes entries.
• All new and modified code is appropriately commented, such that the what and why of its design would be reasonably clear to engineers, preferably ones unfamiliar with the project.
• All tech debt and/or shortcomings introduced by this PR are detailed in TODO and/or FIXME comments, which include a JIRA ticket ID for any items that require urgent attention.
• Reviews are requested from both:
  • At least two other engineers on this project, at least one of whom is a senior engineer or owns the relevant component(s) here.
  • Any relevant engineers on other projects (e.g. BFD, SLS, etc.).
• Any deviations from the other policies in the DASG Engineering Standards are specifically called out in this PR, above.
  • Please review the standards every few months to ensure you're familiar with them.

[dtisza1]

This looks good to me!

I was able to test it out using the BB2 server running locally and it looked good.
I was able to test locally with both the static and web server PRs.