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/trip_model.php

@@ -0,0 +1,9 @@
+<?php
+$date = sanitizeInput($_POST['date']);
+$source = sanitizeInput($_POST['source']);
+$destination = sanitizeInput($_POST['destination']);
+$miles = floatval($_POST['miles']);
+$roundtrip = intval($_POST['roundtrip']);
+$purpose = sanitizeInput($_POST['purpose']);
+$user_id = intval($_POST['user']);
+$client_id = intval($_POST['client']);