api: add server for easy Observable integration (#1537)

Summary:
This commit adds a simple Python server for connecting the output of
`yarn api` (or `yarn api --watch`) to an observable notebook. We need a
custom server rather than just `python3 -m http.server` to send CORS
headers properly. This server enables a very tight loop from editing
SourceCred core code on your local filesystem to seeing live updates in
an Observable notebook, with latency on the order of one second.

Test Plan:
Run `yarn api --watch` in the background. Launch the new API server.
Navigate to <https://observablehq.com/demo>. Copy the two paragraphs of
Observable code from `scripts/serve_api.py` into _separate_ Observable
cells, and execute them. Note that `myGraph` becomes a valid SourceCred
graph. Modify `src/core/graph.js` to add `this._aaa = 123;` to the top
of the `Graph` constructor. Re-execute the first Observable cell (the
one that loads the SourceCred module), and note that `myGraph` updates
to include the new `_aaa` attribute:

![Screenshot of Observable notebook after test plan][ss]

[ss]: https://user-images.githubusercontent.com/4317806/71958748-dddf8680-31a5-11ea-9016-5df76ceeea46.png

wchargin-branch: api-server

scripts/serve_api.py

@@ -0,0 +1,71 @@
+"""Serve SourceCred API, for consumption by Observable notebooks.
+
+Usage:
+
+    python3 ./scripts/serve_api.py [PORT]
+
+from the SourceCred repository root. Omit the port to try a default, or
+specify port 0 to pick a free port automatically.
+
+This server binds to `localhost`, which is a loopback address only
+accessible from your machine, not the local network.
+
+Corresponding Observable code:
+
+    sourcecred = {
+      const server = "http://localhost:9009";
+      const blob = await fetch(server).then((r) => r.blob());
+      const esModule = await require(URL.createObjectURL(blob));
+      return esModule.default;
+    }
+
+    myGraph = new sourcecred.core.graph.Graph()  // e.g.
+
+This server serves the same response for all requests.
+
+For best results, run concurrently with `yarn api --watch`. After
+changing SourceCred code, just re-execute the definition of the
+`sourcecred` module in the Observable notebook to live-load the new code
+and update your notebook.
+"""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import os
+import sys
+from wsgiref import simple_server
+
+
+_API_FILE = os.path.join("dist", "api.js")
+_DEFAULT_PORT = 9009
+
+
+def _app(environ, start_response):
+    # This will 500 if the file can't be read, which is fine.
+    with open(_API_FILE, "rb") as infile:
+        # Re-read the file every time to account for updates.
+        contents = infile.read()
+    headers = [
+        ("Access-Control-Allow-Origin", "*"),
+        ("Content-Type", "application/javascript; charset=UTF-8"),
+        ("X-Content-Type-Options", "nosniff"),
+    ]
+    start_response("200 OK", headers)
+    return (contents,)
+
+
+def _main():
+    port = int(sys.argv[1]) if len(sys.argv) > 1 else _DEFAULT_PORT
+    server = simple_server.make_server("localhost", port, _app)
+    print("Serving on port %d" % server.server_port)
+    try:
+        server.serve_forever()
+    except KeyboardInterrupt:
+        print()
+        pass
+
+
+if __name__ == "__main__":
+    _main()