[560 & 562] One login pre-release banners

[elceebee]

Context

This PR is for two related tickets:
560: A banner displayed to logged in candidates on the personal details and application choices tab to let them know one login is coming
562: A banner displayed on the sign in pages to let candidate users know that one login is coming.

We will put this behind a feature flag -- it will only be displayed when the feature flag is active AND one login is is deactivated.

Changes proposed in this pull request

• Two banner components and related specs and previews
• Changes to the relevant views to display the banners
• Feature flag for toggling banners on / off.

Sign-in page	Details page

Guidance to review

Locally and in the review app, check to see if the banners are displayed as expected. Note that the banner will not display when logged in unless the flash content is empty.

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated.
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card

[elceebee]

One Login: Create service banner for logged-in state | Pre-release

[elceebee]

One Login: Create service banner for Sign-in page | Pre-release

[CatalinVoineag]

Very nice 👍🏻