Call UI disconnect functions before disconnecting with /disconnect

fixes #703

tests/functionaltests/test_disconnect.c

@@ -0,0 +1,24 @@
+#include <glib.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <setjmp.h>
+#include <cmocka.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <stabber.h>
+#include <expect.h>
+
+#include "proftest.h"
+
+void
+disconnect_ends_session(void **state)
+{
+    prof_connect();
+
+    prof_input("/disconnect");
+    assert_true(prof_output_exact("stabber@localhost/profanity logged out successfully."));
+
+    prof_input("/roster");
+    assert_true(prof_output_exact("You are not currently connected."));
+}