Change pybabel keywords from __ to _

[roganw]

Because of fabmanager babel-extract doesn't support -k gettext , string in __(msg) would never be extract to messages.pot.

@cli_app.command("babel-extract")
@click.option('--config', default='./babel/babel.cfg')
@click.option('--input', default='.')
@click.option('--output', default='./babel/messages.pot')
@click.option('--target', default='app/translations')
def babel_extract(config, input, output, target):
    """
        Babel, Extracts and updates all messages marked for translation
    """
    click.echo(click.style('Starting Extractions config:{0} input:{1} output:{2}'.format(config, input, output), fg='green'))
    os.popen('pybabel extract -F {0} -k lazy_gettext -o {1} {2}'.format(config, output, input))
    click.echo(click.style('Starting Update target:{0}'.format(target), fg='green'))
    os.popen('pybabel update -N -i {0} -d {1}'.format(output, target))
    click.echo(click.style('Finish, you can start your translations', fg='green'))

An alternative solution is replace fabmanager babel-extract with pybabel extract in the CONTRIBUTING.md document:
pybabel extract -F ./babel/babel.cfg -k __ -k _ -o ./babel/messages.pot .

[xrmx]

Good catch! You are changing the semantics though. I think we should try to fix the fabmanager command to support more options.

[roganw]

@xrmx Your are right! I will create a fix request to Flask-APPBuilder.
And before it would be merged, I submit a new request to superset CONTRIBUTING.md document to avoid overwriting translations by someone again.
modify of CONTRIBUTING.md