[SalesForce] Add property on lightning component for the community builder

This does work but it's not documented so not really a good idea:

$A.get("$SObjectType.CurrentUser.Id")

The only documented way I think is to call a server side controller action: https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/controllers_server_actions_call.htm

You'd probably want to fetch something like the UserId/ContactId once in your doInit.

I have made a blog about how to resolve this issue without having to add a design attribute. Link: http://kevansfdc.blogspot.com/2017/02/does-lightning-component-know-if-it-is.html

The solution I propose is to use a aura enabled method which return whether you are in a site context using the Site class that come natively with Salesforce.

@AuraEnabled
public static boolean isCommunity(){
    Id siteId = Site.getSiteId(); // take a look at the apex class Site, you may find more useful method concerning site/community
    if (siteId != null) {
        return true;
    }
    return false;
}

And simply calling this method through an action in Lightning.

var action = component.get("c.isCommunity");
action.setCallback(this, function(response) {
var isCommunity = response.getReturnValue(); // do any operation needed here
});
$A.enqueueAction(action);

Hope this is useful as an answer. Cheers!