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_custom_field_model.php

@@ -0,0 +1,3 @@
+<?php
+$label = sanitizeInput($_POST['label']);
+$type = sanitizeInput($_POST['type']);