Resolve "Resize handles for Web IDE right sidebar are too thin"

What does this MR do?

1. Resize the "resize handle" from 1px to 4px.
2. Renames the class dragHandle to drag-handle (according to doc).

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

• This updates the left sidebar as well (which, according to this issue, was also too thin).
• Are these drag* classes used anywhere else? I searched the CE and EE project, but I'd like to make sure it's not coupled to a third-party library or something...

Why was this MR needed?

#48055 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48055 (closed)