make user_id nullable in tags table

This enables support for the system notification feature, since
(system) notification tags does not necessarily belong to any user.

Author: damms005

database/migrations/2020_03_30_075042_make_user_id_nullable_in_tags_table.php

@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+class CreateTagsTable extends Migration
+{
+	/**
+	 * Run the migrations.
+	 *
+	 * @return void
+	 */
+	public function up()
+	{
+
+		Schema::table('tags', function (Blueprint $table) {
+			$table->integer('user_id')->unsigned()->nullable();
+		});
+	}
+
+	/**
+	 * Reverse the migrations.
+	 *
+	 * @return void
+	 */
+	public function down()
+	{
+		$table->integer('user_id')->unsigned();
+	}
+}