WIP: Resolve "New issue button redirects you to create a new issue after logging in"

What does this MR do?

• Update flash message style and message when being redirected to the signin page

Why was this MR needed?

• Current flash message is red (negative) and too general of a message

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #20204 (closed)