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.
To be honest I'm not 100% familiar with the flow of config to the JS app.

I think that's the first case of passing a boolean, but maybe I'm wrong. We should probably fix appConfig to handle booleans, but I think that'll exceed the scope of this PR. What do you think?