Move endpoints to use stream_id instead of stream_name in their URLs

- Change `stream_name` into `stream_id` on some API endpoints that use
`stream_name` in their URLs to prevent confusion of `views` selection.

For example:
If the stream name is "foo/members", the URL would be trigger
"^streams/(?P<stream_name>.*)/members$" and it would be confusing because
we intend to use the endpoint with "^streams/(?P<stream_name>.*)$" regex.

All stream-related endpoints now use stream id instead of stream name,
except for a single endpoint that lets you convert stream names to stream ids.

See https://github.com/zulip/zulip/issues/2930#issuecomment-269576231

- Add `get_stream_id()` method to Zulip API client, and change
`get_subscribers()` method to comply with the new stream API
(replace `stream_name` with `stream_id`).

Fixes #2930.

zproject/urls.py

@@ -293,9 +293,9 @@ v1_api_and_json_patterns = [
         {'GET': 'zerver.views.streams.json_get_stream_id'}),
 
     # GET returns "stream info" (undefined currently?), HEAD returns whether stream exists (200 or 404)
-    url(r'^streams/(?P<stream_name>.*)/members$', rest_dispatch,
+    url(r'^streams/(?P<stream_id>\d+)/members$', rest_dispatch,
         {'GET': 'zerver.views.streams.get_subscribers_backend'}),
-    url(r'^streams/(?P<stream_name>.*)$', rest_dispatch,
+    url(r'^streams/(?P<stream_id>\d+)$', rest_dispatch,
         {'HEAD': 'zerver.views.streams.stream_exists_backend',
          'GET': 'zerver.views.streams.stream_exists_backend',
          'PATCH': 'zerver.views.streams.update_stream_backend',