update textarea height and refocus when attaching files

What does this MR do?

Keep focus in textarea when attaching files Don't add 3 newlines after pasting a file in textarea

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

• not 100% where test for this should go. I don't want to keep adding regression tests to, but directly testing the side effects of the click handler doesn't seem so great either. Suggestions?

Screenshots (if relevant)

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 gitlab-ce#30008