Aligns upload behavior between editors

What does this MR do and why?

The new Rich Text Editor had a very inconsistent behavior when it came to links and uploads, compared to the Markdown Editor. This MR aligns them again:

• Removes "Upload file" from link dropdown
• Changes order of upload button to be behind table
• Makes upload a button again instead of dropdown

See #379045 (comment 1333368596)

Before	After

How to set up and validate locally

Feature.enable(:content_editor_on_issues)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.