Update Wikipedia dataset
This PR combines all updates to Wikipedia dataset.

Once approved, this will be used to generate the pre-processed Wikipedia datasets.

Finally, this PR will be able to be merged into master:
- NOT using squash
- BUT a regular MERGE (or REBASE+MERGE), so that all commits are preserved

TODO:
- [x] #3435
- [x] #3789
- [x] #3825
- [x] Run to get the pre-processed data for big languages (backward compatibility)
- [x] #3958

CC: @geohci