Add a banner to manage.get.gov

We sometimes need to put up a banner on the registrar to inform our users about an event. (Because they are different sites, this banner is different than the one on get.gov, though the content is often the same.)

1. Decide which type of alert is appropriate.

• 🟦 Informative (blue): src/registrar/templates/includes/banner-info.html
• 🟨 Warning (yellow w/exclamation): src/registrar/templates/includes/banner-warning.html
• 🔴 Error (red): src/registrar/templates/includes/banner-error.html
• 🟥 System outage (maroon w/exclamation): src/registrar/templates/includes/system-outage.html
• 🟥 Service disruption (maroon w/exclamation): src/registrar/templates/includes/banner-service-disruption.html
• 🟥 Site alert (maroon w/exclamation): src/registrar/templates/includes/banner-site-alert.html

2. After drafting banner text with the team, modify the appropriate file in a branch.

3. Call the right banner in src/registrar/templates/base.html

• See the comment at https://github.com/cisagov/manage.get.gov/blob/main/src/registrar/templates/base.html#L77-L94 for name and placement.

4. Make a PR

• A PR where we've added a banner can be seen at https://github.com/cisagov/manage.get.gov/pull/4248/files.