Add support for Privacy Act brochure site redirect logging

[aduth]

🎫 Ticket

Supports LG-14093

🛠 Summary of changes

Adds support for the Redirect::PolicyController to redirect with logging to https://login.gov/policy/our-privacy-act-statement/

This is meant to help support LG-14093, which will help identify how many users may exit sign-in using one of the available brochure site links on the Sign In page. Currently, we have no logging insights to clicks on these links.

This updates the "Security Practices and Privacy Act Statement" link to use the redirect controller since support already existed for this, but does not yet update the "Privacy Act Statement" link. That link will be updated in a separate pull request to avoid issues with 50/50 deploy state.

📜 Testing Plan

1. In a separate terminal process, run make watch_events
2. Go to http://localhost:3000
3. Click "Security Practices and Privacy Act Statement"
4. See "Policy Page Redirect" event in events logging with redirect_url of https://www.login.gov/policy/
5. Go to http://localhost:3000/redirect/policy?policy=privacy_act_statement
6. See "Policy Page Redirect" event in events logging with redirect_url of https://www.login.gov/policy/our-privacy-act-statement/

[kevinsmaster5]

Looks good, tests out locally 👍