[Backend Rewrite] Study & Topic

[RobbinBaauw]

• [ ] Recreate all study & topic related endpoints, see https://github.com/finitum/CSHub/tree/dev/cshub-server/src/endpoints/topics and https://github.com/finitum/CSHub/tree/dev/cshub-server/src/endpoints/study. Note that the URLs could have better naming, so feel free to change URLs, HTTP verbs & request/response bodies where you see fit.
• [ ] Add utilities for study & topic to be used throughout the entire application, see for example https://github.com/finitum/CSHub/blob/dev/cshub-server/src/utilities/TopicsUtils.ts (there are more examples of such utilities, please look for those and create a replacement)