[Templating] Investigate error messages in the chrome/firefox dev console for SPAs

[javiercn]

On firefox and chrome, the React and Angular SPA produce a series of error during development.
We need to figure out what's happening in there.

[javiercn]

[ryanbrandenburg]

@mkArtakMSFT we've discussed this issue before. The problem is in JavaScript services, where I don't have any context. @SteveSandersonMS was on vacation the last time it was brought up, perhaps we can discuss it with him now.

[SteveSandersonMS]

I thought the problem was that the dev servers (Angular CLI / create-react-app) emit code that tells the client to make a WebSocket connection back to the dev server. Since we're proxying to the dev server, the WebSocket connection actually goes to the ASP.NET Core host application (which is good and correct), but then somehow we're failing to proxy that WebSocket connection properly.

So my guess is someone needs to debug the proxying code to see why we're not letting the browser and the Angular/React dev server talk to each other in the way they expect.

[SteveSandersonMS]

The error message does contain the clue "Invalid host/origin header", so maybe it's something like: the proxy code needs to rewrite the origin header so that the Angular/React dev server believes the request is coming directly to it, and not from some other origin.