Fixing WordPress Multisite Compatibility Issues in Tutor LMS

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This PR resolves multiple database, schema installation, and page-rendering bugs when running Tutor LMS on a WordPress Multisite network (specifically where subsites use a custom table prefix like wp_2_).

1. Bypassing Subsite Prefixes for Shared Global Tables

• Problem: In a WordPress Multisite, global tables like wp_users, wp_usermeta, wp_blogs, etc., are shared network-wide and do not receive subsite prefixes. Tutor LMS's QueryHelper::prepare_table_name() automatically prepends the subsite table prefix (wp_2_) to all incoming tables, causing SQL execution failures like Table 'wp_2_wp_users' doesn't exist.
• Fix: Updated QueryHelper::prepare_table_name() to recognize all standard global WordPress tables (e.g. $wpdb->users, $wpdb->usermeta, $wpdb->blogs, etc.) and return them unmodified without prepending the prefix.

2. Resolving Hardcoded Foreign Key Constraint Name Conflicts

• Problem: MySQL/InnoDB requires foreign key constraint names to be globally unique within the database schema. Tutor LMS hardcoded constraint names (such as fk_tutor_itemmeta, fk_tutor_ordermeta_order_id, etc.) in its table creation SQL statements. Activating or upgrading the plugin on a second subsite would fail with errno: 150 (Foreign key constraint is incorrectly formed) because the constraint name was already registered by the primary site.
• Fix: Modified Tutor.php and Upgrader.php to dynamically prefix all database constraints with {$wpdb->prefix} (e.g., fk_{$wpdb->prefix}tutor_itemmeta), preventing schema conflicts across multiple sites.

3. Resolving Instructor Dashboard Render Crash

• Problem: The main /dashboard/ page crashed (showing only the header and failing to render the content/footer) because WithdrawModel::get_withdraw_summary hardcoded the query source as FROM {$wpdb->prefix}users u. Since there is no site-specific users table (wp_2_users), the query threw a fatal SQL exception.
• Fix: Corrected the query in WithdrawModel.php to reference the global $wpdb->users table instead.

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Related #2169