Refactor POST handling.

- Split into admin and user handlers, each admin page gets its own file now
- Enforce role access once for admin POST requests
- Automatically load POST logic for admin-based requests based on the referring page, otherwise automatically load all user request logic
- Add support for using custom POST handlers

post/admin/admin_user_model.php

@@ -0,0 +1,5 @@
+<?php
+$name = sanitizeInput($_POST['name']);
+$email = sanitizeInput($_POST['email']);
+$role = intval($_POST['role']);
+$force_mfa = intval($_POST['force_mfa']);