Add toggle to enable/disable joining via coursecode

[k3mystra]

• Refactor coursecode form to auto-submit on either regenerating code or toggling enable/disable joining
• Add stimulus controller to handle the above
• Modify update_coursecode method to handle the new form
• Update the update_coursecode_test integration test to reflect new changes
• Add migration to add coursecode_enabled field in Courses

[vocalfreak]

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