Fixes commit comments in parallel diff view

What does this MR do?

Fixes a JS bug that would prevent comments from being added to commit diffs when in parallel view.

This was caused by the comment button code from checking against the global notes class, which because of the order of code, was undefined. Instead of relying on this global object the comment button code now does its own check for whether the diff is in parallel view or not.

Why was this MR needed?

Screenshots (if relevant)

What are the relevant issue numbers?

Closes #38117 (closed)