Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/examples/application/job/redis/worker.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+
+import time
+import rediswq
+
+host="redis"
+# Uncomment next two lines if you do not have Kube-DNS working.
+# import os
+# host = os.getenv("REDIS_SERVICE_HOST")
+
+q = rediswq.RedisWQ(name="job2", host=host)
+print("Worker with sessionID: " +  q.sessionID())
+print("Initial queue state: empty=" + str(q.empty()))
+while not q.empty():
+  item = q.lease(lease_secs=10, block=True, timeout=2)
+  if item is not None:
+    itemstr = item.decode("utf-8")
+    print("Working on " + itemstr)
+    time.sleep(10) # Put your actual work here instead of sleep.
+    q.complete(item)
+  else:
+    print("Waiting for work")
+print("Queue empty, exiting")