[UI] Fix passing bytes config path to subprocess on Python 3

The % substitution was causing the bytes prefix to become part of the
string and created a `b'/` prefixed config directory. Ensure the config
arg is byte prefixed too.

deluge/ui/client.py

@@ -671,7 +671,7 @@ class Client(object):
         # subprocess.popen does not work with unicode args (with non-ascii characters) on windows
         config = config.encode(sys.getfilesystemencoding())
         try:
-            subprocess.Popen(['deluged', '--port=%s' % port, '--config=%s' % config])
+            subprocess.Popen(['deluged', '--port=%s' % port, b'--config=%s' % config])
         except OSError as ex:
             from errno import ENOENT