LG-9872: Add missing interpolation value for personal key alert

[aduth]

🎫 Ticket

LG-9872

Fixes #8443

🛠 Summary of changes

Fixes the notification sent when signing in using Personal Key as the multi-factor authentication method, which previously included the literal %{app_name} rather than "Login.gov".

As a proactive step to try to identify these sorts of issues sooner, the changes also include a monkeypatch to the I18n.t method when tests are run, raising an exception if the result of the translation includes a lingering interpolation variable.

📜 Testing Plan

(Note that this is difficult to test in deployed environments because it relates to a legacy behavior where "personal key" was supported as an MFA method without an existing identity-verified account)

1. Go to http://localhost:3000/
2. Create an account
3. Once account creation is complete, go to http://localhost:3000/verify
4. Complete identity verification
5. Download or make a copy of the personal key
6. When returned to the account dashboard, click "Forget browsers" in the sidebar and confirm
7. Sign out
8. In your terminal, open a Rails console with rails console
9. Delete the identity verified profile by entering User.find_with_email('<email here>').profiles.delete_all (substitute "<email here>" with your chosen email)
10. Sign in
11. When prompted for MFA, click "Choose another authentication method"
12. Click "Personal key"
13. Click "Continue"
14. Enter the personal key you copied from the identity verification process
15. Click "Submit"
16. Go to http://localhost:3000/test/telephony
17. Observe that the sent message includes "Login.gov", not "%{app_name}"

[matthinz]

LGTM, one possibility for adding a new test