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/event_model.php

@@ -0,0 +1,10 @@
+<?php
+$calendar_id = intval($_POST['calendar']);
+$title = sanitizeInput($_POST['title']);
+$location = sanitizeInput($_POST['location']);
+$description = sanitizeInput($_POST['description']);
+$start = sanitizeInput($_POST['start']);
+$end = sanitizeInput($_POST['end']);
+$repeat = sanitizeInput($_POST['repeat']);
+$client = intval($_POST['client']);
+$email_event = intval($_POST['email_event']);