WIP: Resolve "Make search boxes consistent throughout GitLab"

What does this MR do?

• Add magnifying glass to Select2 Search Box when empty
• Add clear Icon to Select2 Search Box when full
• Add clear Icon to standard input boxes that do a search afterwards

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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 #26303 (moved)