Make emails unique

Emails are made unique. When user attempts to sign in through a provider in which his email is one that is already registered, user is redirected to the signin page with an error passed as a query string parameter.

modules/users/server/models/user.server.model.js

@@ -52,6 +52,7 @@ var UserSchema = new Schema({
 	email: {
 		type: String,
 		trim: true,
+		unique: true,
 		default: '',
 		validate: [validateLocalStrategyEmail, 'Please fill a valid email address']
 	},