adding wrapper for proxy header passing.

add a selection to the proxy editing page and passes that down into the templates.
Removed set_proxy_header from locations and moved to server directive. these will inherit down into locations if they are not defined there.

frontend/js/models/proxy-host.js

@@ -19,6 +19,7 @@ const model = Backbone.Model.extend({
             hsts_subdomains:         false,
             caching_enabled:         false,
             allow_websocket_upgrade: false,
+            forward_proxy_header:    true,
             block_exploits:          false,
             http2_support:           false,
             advanced_config:         '',