Add refactored scripts for Ex3

Esther Vogtrequested to merge
review/ex3-t2-rev-ev into development

Added refactoring to:

  • R/T: frontend/www/scripts/gallery.js
  • R/T: frontend/www/scripts/exercise.js
  • R/T: backend/secfit/users/views.py Additionally: Added/Updated corresponding test scripts

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