I'm pretty sure specgen uses the file to generate bucket2index in histogram_specs.json for dashboard.py...
It would probably be better if histograms_specs.json where generated by dashboard.py at initialization using Histograms.json

I put this in `mr2disk.py` because it is the file that generates the contents of the `data/` folder...

Also I don't quite understand why the makefile saves it in `validation/nightly/25.0a1/histogram_descriptions.json` this makes very little sense...

I suggest that:
  * we let `makefile` download it as `Histogram.json`
  * pass it to `specgen.py` the way we do now...
  * Read it from `Histogram.json` inside `mr2disk.py`, so this file doesn't have to download it... but it still saves it to `data/` folder..

What do you think?