Disable "transfer group" button when no parent group is selected

What does this MR do?

It disables the "Transfer group" button in the settings page of a group when no (new) parent group to transfer to is selected (and "No parent group" isn't selected either) as requested in #64799 (moved).

We have something similar in place for "Transfer project" and I decided to refactor the existing code (app/assets/javascript/project_edit.js) to be more flexible and reusable in an attempt to reduce the need for duplicate code.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process. (I hope this is sufficient)
• [] Tested in all supported browsers (shouldn't be needed)

Closes #64799 (moved)