Handle subscription of messages in the webapp.

Messages larger than a threshold are split and reassembled due to the
size limit in webrtc. Threshold may have to be adjusted somewhere between
64KiB and 256KiB.

This also includes a basic handler for a ping message and a more
advanced image handler.

Change-Id: If66acfb1bb84e9d3ff686994a94b1480cb70b2aa
diff --git a/y2020/BUILD b/y2020/BUILD
index 1d281bb..c80550b 100644
--- a/y2020/BUILD
+++ b/y2020/BUILD
@@ -125,3 +125,15 @@
     srcs = ["__init__.py"],
     visibility = ["//visibility:public"],
 )
+
+sh_binary(
+    name = "web_proxy",
+    srcs = ["web_proxy.sh"],
+    data = [
+        ":config.json",
+        "//aos/network:web_proxy_main",
+        "//y2020/www:main_bundle",
+        "//y2020/www:files",
+        "//y2020/www:flatbuffers",
+    ],
+)