Consolidate all translation config

[alanmcruickshank]

Fixes #3745

Move all translation config to superset/translations, as suggested in the above issue. The babel folder which has been removed was being used as a default, hence the update to the docs to show that this now needs to be specified explicitly.

[coveralls]

Coverage remained the same at 71.271% when pulling dbd6a0a on alanmcruickshank:consolidate_translations into 5bc734b on apache:master.

[xrmx]

The path would be valid only if you installed with npm install -g. The package in debian / ubuntu looks the correct one to latest release 0.4.5 as we have in package.json. What was the error?

[alanmcruickshank]

I did install using npm install -g, as that's what it currently recommends in CONTRIBUTING.md. Should we update that to not recommend a global install?

I wonder if actually, I should update the docs here to add clarity around what needs to be done to makes sure po2json is installed in the right place more broadly. I found the process quite fiddly for someone who doesn't use node very often - global vs local installs and what folder things are available in was at the root of that I think. I'd love to make the process as easy as:

1. Run command X in folder Y to install Z.
2. Run command A in folder B to convert messages.

[xrmx]

It docs use global install then it's fine.

[mistercrunch]

Thanks for adjusting this. Eventually we can wrap those in npm and superset CLI commands. Though these operations are not very common so it's not too important.