Resolve "Have mailto: link in "Email a new issue to this project""

What does this MR do?

Adds a "mailto" button to create issues / MRs through email.

Followup of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10942

Normal screen sizes

Mobile screen sizes

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #30665 (closed)