Speed up loading of comparison page

[Kr0nox]

Currently the submissionFileIndex is just one big file containing information about all files of all submissions. This file can be pretty large for some datasets. Since the entire file has to be read and parsed when opening a comparison this can lead to slowdowns. There are two possbile improvements for this:

• [x] Only write submissions into the index that are part of an exported comparison. Depending on the dataset and the amount of exported comparisons this can reduce the size of the index file significantly. This change can be conisdered non breaking, since it would be backwards compatibile, allthough technically there is less information in the report than previously.
• [ ] Write an index file per submission. This would take the load from reading one big file to only reading two short files. This would change the report thus could be considered breaking, but it could be implemented backwards compatible, by doing one thing when the big index file is present and another when the short ones are