Resolve "QA: Merge request squash sometimes fail"

What does this MR do?

This ensures we wait for the squash checbox to be clickable before clicking it in the squash scenario.

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

1. We now reload the page as long as the MR widget shows the "Please reload the page and review the changes before merging" message.
2. The squash checkbox is disabled on page load, then enabled via JS, so we just wait without reloading that the checkbox is enabled, before checking it.

Why was this MR needed?

Because the squash scenario was very flaky.

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48779 (closed)