Resolve Stop saving generated pdfs

Closes #472 (closed)

In this MR Zesje stops saving generated pdfs, they are stored in memory and send as a response.

Other changes:

• Moved pdf generation logic from api/exams to pdf_generation.
• Constants like exam file name and directory are retrieved from a single function instead of defining them each time.
• Added documentation and new tests for most functions in pdf_generation.