rename_stream.py: Fix broken code.

* get_realm returns None if no matching realm is present, but
  create_stream.py assumed it raises Realm.DoesNotExist.
* encoded/decode strings properly.

tools/run-mypy

@@ -41,7 +41,6 @@ exclude_py2 = []
 
 exclude_py3 = """
 zerver/lib/ccache.py
-zerver/management/commands/rename_stream.py
 zerver/management/commands/runtornado.py
 zerver/tests/test_i18n.py
 zerver/views/messages.py

zerver/management/commands/rename_stream.py

@@ -7,6 +7,7 @@ from argparse import ArgumentParser
 from django.core.management.base import BaseCommand
 
 from zerver.lib.actions import do_rename_stream
+from zerver.lib.str_utils import force_text
 from zerver.models import Realm, get_realm
 
 import sys
@@ -30,11 +31,10 @@ class Command(BaseCommand):
         new_name = options['new_name']
         encoding = sys.getfilesystemencoding()
 
-        try:
-            realm = get_realm(domain)
-        except Realm.DoesNotExist:
+        realm = get_realm(force_text(domain, encoding))
+        if realm is None:
             print("Unknown domain %s" % (domain,))
             exit(1)
 
-        do_rename_stream(realm, old_name.decode(encoding),
-                         new_name.decode(encoding))
+        do_rename_stream(realm, force_text(old_name, encoding),
+                         force_text(new_name, encoding))