Changes the unique constraint for contacts (#203)

Changes the unique constraint for contacts from 'email' to ['account_id', 'email'] (#203)

I think the unique constraint is too eager for the contacts table. 

If a user wants to add a contact with an already existing email, they'll encounter an error. Since they aren't the one who added that email to their contacts, it will be confusing & frustrating. 

I think this is a bug and the constraint should be `['account_id', 'email']`.

database/migrations/2017_06_11_110735_change_unique_constraint_for_contacts.php

@@ -0,0 +1,36 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class ChangeUniqueConstraintForContacts extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('contacts', function (Blueprint $table) {
+            $table->dropUnique(['email']);
+
+            $table->unique(['account_id', 'email'], 'unique_for_each_account_email_pair');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('contacts', function (Blueprint $table) {
+            $table->dropUnique('unique_for_each_account_email_pair');
+
+            $table->unique('email');
+        });
+    }
+}