Initial commit of voting feature.

This makes it possible for users to vote on whether they approve or disapprove of terms of service. The code is not yet completely finished, I just wanted to get some initial feedback on the feature idea. But the functionality currently works. I'm using my Firebase account as a backend, but if the feature were adopted it could be switched to a Firebase account of anybody officially on the tosdr project. I started a [Google Groups discussion](https://groups.google.com/forum/#!topic/tosdr/uobbgDho3Ko) about this. 

Thanks for this awesome project :)