add APIs for using course/exercise slugs instead of ids in tmc-langs

[Heliozoa]

blockers

• download-or-update-course-exercises
  • get /api/v8/core/exercises/details
• get-course-data
  • get /api/v8/core/courses/{id}
• get-course-details
  • get /api/v8/core/courses/{id}
• get-course-exercises
  • same return type from /api/v8/courses/{id} and /api/v8/org/{org}/courses/{course}/exercises (see https://tmc.mooc.fi/api/v8/courses/600 and https://tmc.mooc.fi/api/v8/org/mooc/courses/java-programming-i/exercises)
• get-exercise-details
  • get /api/v8/core/exercises/{id}
• get-exercise-submissions
  • get api/v8/exercises/{exercise_id}/users/current/submissions
• get-exercise-updates
  • get /api/v8/core/courses/{course_id}