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
Now that we're mixing "all grunt tasks in Gruntfile.js" and "load some external ones from grunttasks/", does it make sense to start breaking up the Gruntfile and moving the individual tasks into the grunttasks/* directory? Or want to leave as-is for now?