Prevent students from changing their group

Fix for https://github.com/learningequality/ka-lite/issues/2414

Uses javascript to disable group select box when a student is editing their account, while preserving ability for students to edit groups when they create or edit accounts. This functionality can be further cleaned up when I address https://github.com/learningequality/ka-lite/issues/2391 but for now this works fine and can get in quickly for release 0.12 and nalanda