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/user/product_model.php

@@ -0,0 +1,6 @@
+<?php
+$name = sanitizeInput($_POST['name']);
+$description = sanitizeInput($_POST['description']);
+$price = floatval($_POST['price']);
+$category = intval($_POST['category']);
+$tax = intval($_POST['tax']);