exercises api

[toasted226]

the following services are implemented
user-courses

• assigning and removing courses for a user
• getting courses for a user
  practice-sessions
• insert practice session
  -- missing deletion
  exercises-completed
• insert single completed exercise
• insert all completed exercises from practice session
• generic query function for exercises-completed

endpoints needed:
student/session/start - pull exercises to be given to the user
student/session/end - post session stats (see excalidraw payload)
student/courses/:id/assign - assign course to user
student/courses/:id/remove - remove course from user