Fix download of task files

[patsalv]

Currently, file downloads only work properly if all the files have been opened at least once in Task.tsx.

Reproduction:

1. visit multi-file task, e.g. https://access-staging.ifi.uzh.ch/courses/info1-hs25-staging/assignments/inheritance/tasks/rpg_fights
2. edit mage.py, add superclass: class Mage(Character)
3. Run the code to save a snapshot
4. Reload the page -> knight.py will be open by default
5. Click download button in file tree -> the resulting zip file contains mage.py template (without inheritance) instead of our version
6. Click mage.py in the file tree
7. Click download button again -> resulting zip contains correct mage.py