Setup SSO for Admin app

[phil-l-brockwell]

📝 A short description of the changes

• Replace existing authentication for Admin app with OmniAuth and integrate with the DBT Staff SSO

🔗 Link to the relevant story (or stories)

• Asana

Deployment implications

• Integrate with the DBT staff SSO once we have some test credentials before merging (tested in development enviroment with an instance of: doorkeeper.
• Can we drop the redundant authorisation db fields now? eg: encrypted_password/authy_id/authy_enabled/last_sign_in_with_authy/etc

✅ Checklist

• Features that cannot go live are behind a feature flag/env var or specify deploy date and open PR as draft
• I have checked that commit messages make sense and explain the reasoning for each change
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have squashed any unnecessary or part-finished commits

🖼️ Screenshots (if appropriate - no PII/Prod data):

To fix the CSRF vulnerability, we should re-enable CSRF protection for the developer action. This can be done by removing the skip_before_action :verify_authenticity_token, only: :developer line. Additionally, we can add a safeguard to ensure that the developer action is only accessible in development environments by using a more secure approach, such as an environment-specific route constraint.