Copy `start` to `start-v3` (#471)

Summary:
This could also be moved into the bridge directory, but this way is
marginally easier, and it doesn’t really matter in the end.

Test Plan:
`yarn backend` followed by `node bin/sourcecredV3.js start-v3` works.

wchargin-branch: start-v3

config/paths.js

@@ -62,6 +62,7 @@ module.exports = {
     //
     sourcecredV3: resolveApp("src/v3/cli/sourcecred.js"),
     "commands/load": resolveApp("src/v3/cli/commands/load.js"),
+    "commands/start-v3": resolveApp("src/v3/cli/commands/start.js"),
     //
     fetchAndPrintGithubRepo: resolveApp(
       "src/v1/plugins/github/bin/fetchAndPrintGithubRepo.js"

src/v3/cli/commands/start.js

@@ -0,0 +1,72 @@
+// @flow
+
+import {Command} from "@oclif/command";
+import chalk from "chalk";
+import fs from "fs";
+import {choosePort} from "react-dev-utils/WebpackDevServerUtils";
+
+import apiApp from "../../../bridge/app/apiApp";
+import {sourcecredDirectoryFlag} from "../common";
+
+const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 4000;
+const HOST = process.env.HOST || "0.0.0.0";
+
+export default class StartCommand extends Command {
+  static description = "start a web server to explore the contribution graph";
+
+  static flags = {
+    "sourcecred-directory": sourcecredDirectoryFlag(),
+  };
+
+  async run() {
+    const {
+      flags: {"sourcecred-directory": sourcecredDirectory},
+    } = this.parse(StartCommand);
+    startServer(sourcecredDirectory);
+  }
+}
+
+async function startServer(sourcecredDirectory: string) {
+  let server;
+  function cleanup() {
+    if (server && server.listening) {
+      server.close();
+    }
+  }
+
+  let shuttingDown = false;
+  ["SIGINT", "SIGTERM"].forEach((signal) => {
+    process.on(signal, () => {
+      if (shuttingDown) {
+        // Force shut-down.
+        process.exit(2);
+      } else {
+        shuttingDown = true;
+        console.log("\nShutting down.");
+        cleanup();
+      }
+    });
+  });
+
+  const staticFiles = "./build/";
+  if (!fs.existsSync(staticFiles)) {
+    console.error("Build output not found. Did you run `yarn build`?");
+  }
+
+  console.log(chalk.bold("Starting Express..."));
+  const expressApp = apiApp(sourcecredDirectory, staticFiles);
+  server = await new Promise(async (resolve, _unused_reject) => {
+    const port = await choosePort(HOST, DEFAULT_PORT);
+    let server = expressApp.listen(port, () => {
+      resolve(server);
+    });
+  });
+  server.on("close", () => {
+    console.log(chalk.bold("Express server closed."));
+    cleanup();
+  });
+  console.log(
+    chalk.green(`Server listening on port ${server.address().port}.`)
+  );
+  console.log();
+}