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/transfer_model.php

@@ -0,0 +1,7 @@
+<?php
+$date = sanitizeInput($_POST['date']);
+$amount = floatval($_POST['amount']);
+$account_from = intval($_POST['account_from']);
+$account_to = intval($_POST['account_to']);
+$transfer_method = sanitizeInput($_POST['transfer_method']);
+$notes = sanitizeInput($_POST['notes']);