Force client side HMR inference for out-of-the-box HTTPS support

[TobbenTM]

The current implementation to achieve HTTPS is far from ideal, and this PR will do what is more aligned with how the other SPA proxies work, namely force the client webpack script to use this SPA proxy to also proxy HMR websocket connections.

As seen above, this is how it currently works; since the HMR endpoint is hard-coded to point to the Vue CLI server, it doesn't use the correct protocol.

With this PR we force the browser to use the current host as HMR endpoint, and just pass the websockets through the same proxy we use for other requests.
This lets us skip setting up Vue CLI with certificates, just try out the sample, it is now using HTTPS by default.

I'd wish we had a better way to force this inference than the magical environment variable, there is a fairly old issue for Vue CLI here: vuejs/vue-cli#3620

[bryandam]

FWIW: I tested the VueCliSample project from this PR and confirmed it worked. I also incorporated it into my project to resolve HMR Reload. (Issue #72). Great work @TobbenTM!

[EEParker]

Thank you for the PR!