We were using a custom icon because in the past social-share-button
didn't have support for whatsapp. But now that it does, we can remove
our custom icon.
Note we're using the `_app` suffix because that's the name of the icon
meant for mobile devices.
* Add the social buttons gem to the Gemfile
* Initializer and translations for the social buttons
* Add the social buttons assets to the sprockets require
* Add the social buttons in the show debate view
* Spec to lock down the amount of social share buttons to three
