Make the inventory backend standalone runnable

And also make a proposed split of how the loggers are handled.

Now logs look like

```
{
  "type": "incomingRequest",
  "service": "backstage",
  "timestamp": "2020-04-30T14:29:04.002Z",
  "message": "...",
  "level": "info"
}
```

for requests, and then if the plugin logs something properly

```
{
  "type": "plugin",
  "service": "backstage",
  "plugin": "inventory",
  "timestamp": "2020-04-30T14:29:04.002Z",
  "message": "...",
  "level": "info"
}
```

packages/backend-common/README.md

@@ -24,8 +24,8 @@ import {
 const app = express();
 app.use(requestLoggingHandler());
 app.use('/home', myHomeRouter);
-app.use(errorHandler());
 app.use(notFoundHandler());
+app.use(errorHandler());
 
 app.listen(PORT, () => {
   getRootLogger().info(`Listening on port ${PORT}`);