clean up CI

[hleepfl]

Overall update of the CI workflows. I recommend reviewing by commit.
Main highlights :

1. Repetitive actions have been extracted to custom actions
2. Main CI workflow was broken up into 1 main file and 3 sub files. It now runs in order : static code analysis > build > tests, with each step requiring the previous one. It's clean but a little bit slower, as we only start the tests after building everything. I believe that the previous version was incorrect in not waiting for webapp to be built before running though.
3. Dependency graph between build jobs is simpler to read
4. TSConfig references have been fixed, which allows linting to run without having to build anything. This does add a little bit of code duplication. sad.
5. Test jobs have been split to make it clearer which are failing or flaky and allow more targeted re-runs. No gains in speed, and we do end up running the setup actions more in the end, but I believe the added clarity is worth it
6. Dataset downloads were using cache to do both the regular cache job, but also artifacts (passing data between jobs inside a workflow). Concerns are now split, and the hash key is the download script (I assumed target files are static, but either way the previous hash key did not do anything either I think)
7. Build artifacts are now transferred between builds and tests. This adds a lot of ugly code and gives only a ~10s gain on test jobs. Should we just remove it ? It will probably be more useful once we have Rust code
8. Checks will not be run anymore on Markdown-only changes. It should not block merging but I haven't tested it.

Conclusion : CI is more readable (imo) and modular. But there are close to zero efficiency gains apart from the markdown filter thing. Thanks for reading.