Change qa-* class references to js-* for squash-checkbox

What does this MR do?

Recently, master was broken because a qa selector class was removed from an element while an rspec still referenced it. There are a couple dozen more locations where this could happen and these are potential future breakages as Quality shifts from using qa classes to data selectors. We should decouple rspec and js dependence on qa selectors.

This MR shifts js use of the class qa-squash checkbox to js-squash-checkbox and adds that class onto the appropriate elements.

Issue aggregating these fixes: https://gitlab.com/gitlab-org/quality/team-tasks/issues/182

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
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• [-] Merge request performance guidelines
• [-] Style guides
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• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team