wms proxy

Down the rabbit hole, a suggested fix for #946. I saw that authenticated geoserver requests go via the proxy controller, so why not just run them all through?

Revised a few more times by @dnahodil and @jkburges, looks mature now - has flag to enable/disable the feature.
I seem to remember there being a problem with JSON encoding/decoding somewhrere along they way. And so it turned proper booleans (and probably proper numbers, too) into strings. I thought you might have come across it.

It's not the end of the world and hopefully that config will be changing soon, anyway. I'm happy to leave this.