Update alerts in mailers to align to match design system

[aduth]

🛠 Summary of changes

Updates alert content in mailers to align to the new design system styling introduced in 18F/identity-design-system#439 .

📜 Testing Plan

1. Go to http://localhost:3000/rails/mailers/user_mailer/in_person_ready_to_verify
2. Observe alert appears visually consistent with other alerts in application (example)

Extra:

• To simulate outage alert visible, you can change condition on this line to always be true (e.g. || true)

👀 Screenshots

Before	After

[aduth]

One thing I noticed is that we use mostly default alert markup in report mailers (example, only overriding image to avoid unsupported SVG). That might make things a little simpler to maintain, at the cost of including more styles in every email that we send.

[aduth]

A couple other notes:

• This also fixes an issue where the fraud reports were using the old icon, but otherwise new styles (before, after)
• This isn't fully consistent with design system alerts (particularly border radius), but (a) it's close enough without having to delve into complex styling and (b) those inconsistencies existed previously anyways

[eileen-nava]

Thanks for doing this, it looks good! I'm including a screenshot of the enhanced ipp reminder email for posterity.

Enhanced IPP Reminder Email:

[gina-yamada]

I also let @rutvigupta-design know about the change so that our mocks can be updated to match this change.