Rework return_path session key handling

- Add new IHandleSessions::pop() method
- Remove redirection from Authentication::setForUser()
- Add explicit return_path form parameter to Login::form()

src/Core/Session/Capability/IHandleSessions.php

@@ -54,6 +54,16 @@ interface IHandleSessions
 	 */
 	public function get(string $name, $defaults = null);
 
+	/**
+	 * Retrieves a value from the provided key if it exists and removes it from session
+	 *
+	 * @param string $name
+	 * @param mixed  $defaults
+	 *
+	 * @return mixed
+	 */
+	public function pop(string $name, $defaults = null);
+
 	/**
 	 * Sets a single session variable.
 	 * Overrides value of existing key.