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/xcustom/readme.php

@@ -0,0 +1,13 @@
+<?php
+
+/*
+    - Custom Pages -
+
+    If you wish to add custom pages to ITFlow, add them to the root directory with the prefix "xcustom_"
+        e.g. If your page was called my_page_one, name it "xcustom_my_page_one.php"
+    Note: If required, you can use the Custom Links module to have the page show on the user sidebar.
+
+    To process POST data via your custom pages, create a file in this directory (post/xcustom) named after your page (e.g. xcustom_my_page_one.php).
+    The relevant file will be automatically loaded upon a POST request based on the referer - your form just needs to target the standard post.php.
+
+ */