Use a more robust method of disabling collapsing the publish box

[helen]

Description of the Change

Instead of removing the postbox class which has side effects beyond the intention, specifically target the arrow (handlediv) and remove the click handler that allows collapsing the metabox.

Alternate Designs

n/a

Benefits

No more UI cruft.

Possible Drawbacks

Possible for it to stop working in a future core update, as already happened with the previous method. However, it should not cause UI cruft.

Verification Process

Manual testing.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Fixes #36

[adamsilverstein]

Nice work!