Specify maximal length in translation

How it was found

Hi,

It would be nice if you limit the length in translation (the same problem here https://hackerone.com/reports/223931).

You can see the "spam" here: https://demo.weblate.org/translate/hello/master/gl/?sid=c8c733d6-2a6b-11e7-9c94-5254000d9bae&offset=3

Thanks