Replace STUDYPATH with the built-in STUDYSUBJECT
Do not translate studysubjects (does not make any sense)
Revert 9df5d3: Remove the now unused studypath user property
Clean up core: remove all occurrences of studypath
Delete empty studypath translation files
Remove hardcoded studypaths from german translation file
Complete refactoring of the StudyPath class: save the name instead of I18n keys
Update database schema to reflect latest StudyPath changes
Fix some remaining occurrences of i18nKey
Merge: Use builtin studypath field, fixes #7