Separate generation of short_urls from their saving

Split ``Displayable.set_short_url`` into two methods, one responsible for asking ``bit.ly`` for a new URL and another saving it as ``short_url``.

This might be useful if you would need a one-time shortened URL for a model and didn't want to save it or would like to provide an alternative generation using some other service. But most importantly, it [lets](https://github.com/wrwrwr/mezzanine/compare/feature/modeltranslation-integration-1.7#diff-1c58d63f030c4e81caa29515df94db00L262) my modeltranslation integration generate a ``short_url`` for each language the model is available in :-)