Revert removal of inbuilt proxy functionality

[chancecarey]

This was incredibly useful to have built in and mentioned in the config file.

Now it's not actually mentioned in the reference config at all, only in the guide, and then it requires that you install http-proxy, which will realistically also require http-proxy-middleware to do things like rewrite URLs to remove an /api prefix. Functionality that used to be built in :(

Webpack has proxying built in, as does parcel.

This was very useful functionality, I'd like to see it brought back :)