bots: Move contrib_bots to api/bots*.

This will make it convenient to include these bots in Zulip API
releases on pypi.

Fix #5009.

api/bots/helloworld/helloworld.py

@@ -0,0 +1,18 @@
+# See readme.md for instructions on running this code.
+
+
+class HelloWorldHandler(object):
+    def usage(self):
+        return '''
+        This is a boilerplate bot that responds to a user query with
+        "beep boop", which is robot for "Hello World".
+
+        This bot can be used as a template for other, more
+        sophisticated, bots.
+        '''
+
+    def handle_message(self, message, client, state_handler):
+        content = 'beep boop'
+        client.send_reply(message, content)
+
+handler_class = HelloWorldHandler