fixed duplicate API calls when showing previews for exercises from the exercise list page

mc_backend/mcserver/app/__init__.py

mc_backend/mcserver/app/logs/.gitignore

mc_backend/mcserver/app/services/corpusService.py

mc_backend/mcserver/config.py

mc_backend/tests.py

mc_frontend/src/app/exercise-list/exercise-list.page.spec.ts

mc_frontend/src/app/exercise-list/exercise-list.page.ts

mc_frontend/src/app/exercise.service.spec.ts

mc_frontend/src/app/exercise.service.ts

mc_frontend/src/app/preview/preview.page.ts