Announce Federalist changing to cloud.gov Pages on app and docs

[apburnes]

User Story

When a user is on federalist.18f.gov or federalistapp.18f.gov, a banner announcing the name change and move under cloud.gov will be visible with links to the migration info page

Background (Optional)

Inform our users about the upcoming change

Acceptance Criteria

• [x] Add banner to app
• [x] Add banner to marketing site home page
• [x] Create page within docs about migration to marketing site