src/email.php: Remove unused namespace import.
src/test/webapp.php: Add comment to method.
src/webapp/: Change interface for filters a tad, allowing a filter to return a response (string or HttpResponse object) if it wishes to terminate a request prematurely.
src/webapp/: Add ui.php, which so far just contains a method for generating a decent-looking HTML-based error alert.
src/webapp/filters/csrf-guard.php: Change the way invalid CSRF tokens are handled, giving back a decent-looking validation error instead of treating it as a generic malicious request (by raising a MaliciousRequestException).