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/invoice_model.php

@@ -0,0 +1,8 @@
+<?php
+$date = sanitizeInput($_POST['date']);
+$category = intval($_POST['category']);
+$scope = sanitizeInput($_POST['scope']);
+$invoice_discount = floatval($_POST['invoice_discount']);
+$recurring_discount = floatval($_POST['recurring_discount']);
+
+$config_invoice_prefix = sanitizeInput($config_invoice_prefix);