devtools: add a script to fetch and postprocess translations

Run this script from the root of the repository to update all translations from transifex.
It will do the following automatically:

- create a transifex configuration file
- fetch all translations
- post-process them into valid and committable format
Full translation update

Last update (48be9ce) missed quite a lot, for some reason.

This is also the first update done with the new script
`contrib/devtools/update-translations.py`