feat: notification unsubscribe & email overhaul with HTML multipart (#3872)

framework/core/migrations/2023_08_19_000000_create_unsubscribe_tokens_table.php

@@ -0,0 +1,30 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+return Migration::createTable(
+    'unsubscribe_tokens',
+    function (Blueprint $table) {
+        $table->id();
+        $table->unsignedInteger('user_id');
+        $table->string('email_type');
+        $table->string('token', 100)->unique();
+        $table->timestamp('unsubscribed_at')->nullable();
+        $table->timestamps();
+
+        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+
+        $table->index('user_id');
+        $table->index('email_type');
+        $table->index('token');
+        $table->index(['user_id', 'email_type']);
+    }
+);