Update email style for vulnerabilities and dependencies export

What does this MR do and why?

Update email style for vulnerabilities and dependencies export.

• Add download button
• Update email subject
• Differentiate the format between Security Dashboard and Vulnerability Report emails

References

• Reformat email sent for exported vulnerability ... (#554247 - closed)

Screenshots or screen recordings

Dependency List:

Before	After
Screen_Recording_2025-07-25_at_14.40.38	Screen_Recording_2025-07-25_at_14.42.14

Security Dashboard:

Before	After
Screen_Recording_2025-07-25_at_14.41.16	Screen_Recording_2025-07-25_at_14.42.37

Vulnerability List:

Before	After
Screen_Recording_2025-07-25_at_14.41.16	Screen_Recording_2025-07-25_at_14.56.57

How to set up and validate locally

1. Go to http://gdk.local:3000/<project>/-/security/vulnerability_report and click the export button
2. Open http://gdk.local:3000/rails/letter_opener/ and check for email
3. Repeat for http://gdk.test:3000/groups/<group>/-/dependencies
4. Repeat for http://gdk.test:3000/groups/<group>/-/security/dashboard

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.