WIP: L10n string export and import

- [x] Export strings from mustache files
- [x] Export strings from js files
- [x] Export strings from go templates
- [x] Import po files from external repo
- [x] Copy and convert po files into json
- [x] Move FindMyDevice-l10n to mozilla-services and update reference
We need [the features](https://github.com/shane-tomlinson/grunt-po2json/commits/custom-filenames) that Shane added to customize the output.