Redirect to sign_in page for magic link pages if one-login enabled

[CatalinVoineag]

Context

When the one login feature flag is activated, we should not let the user to go to any magic links urls

Changes proposed in this pull request

before check in magic links controllers

Guidance to review

Go on review app and try these URLs, you should be redirected back to sign in page

/candidate/sign-up
/candidate/sign-in

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

[CatalinVoineag]

https://trello.com/c/8VLDNNqw/509-implement-one-login-authentification-with-content

[elceebee]

Thanks for cleaning this up! I've tested on the review app and it works as expected