Add "centered" banner component variant

[aduth]

🛠 Summary of changes

Formalizes the "centered" variant of the banner component used in the IdP, and adds documentation for banner component usage.

This also includes revisions and recommendations that build upon the default USWDS documentation, slated for upstream pull requests:

• Add width and height attributes to all images (related blog post, avoids common automated scanner issues)
• Use us_flag.svg (vector) instead of us_flag_small.png (raster) for improved scalability, smaller file size
• Replaces inline SVG element with reference to icon.svg as <img /> element

📜 Testing Plan

1. npm start
2. Visit http://localhost:4000/banner/
3. Verify banner appears as expected, both "Default" and "Centered" variants, including recommended markup

👀 Screenshots

[zachmargolis]

this is a lot of repeated stuff for just the id and class to be different, do we ever consider making these into partials w/ variables?

2c2
<   class="usa-banner"
---
>   class="usa-banner usa-banner--centered"
28c28
<           aria-controls="gov-banner-default"
---
>           aria-controls="gov-banner-centered"
36c36
<       id="gov-banner-default"
---
>       id="gov-banner-centered"

[aduth]

Yes! It was made difficult to propose with how I split up the pull requests, but part of what I'm hoping with #433 is to just {% include banner.html %} for these component examples and have some variable (or auto-incrementing number) to create unique IDs.

Reflected in that PR message:

This also opens the possibility of internally reusing "includes" for header with documented usage for guaranteed one-to-one documentation and usage

[aduth]

I'll try to arrange merge order of the pull requests so that I can rebase to include this improvement.

[aduth]

Rebased and updated to reuse banner.html in 4b3fcca.

[aduth]

Visual regression failure expected for the added navigation link.