UI: Enable sticky proxy for eventing

Task is to implement below for Eventing. I'm creating it as a bug because needing to connect to a specific node to access specific functionality has always a jarring experience.

======
FYI, in the master (mad-hatter) branch, it is now possible to configure your UI plug-ins to work whether or not the node has the necessary service. In each UI plug-in, there is a file called something like, “pluggable-ui-<name>.json.in”. In it, there is a line:

"proxy-strategy": "local",

Change this to:

"proxy-strategy": "sticky",

And you will be able to access your functionality from any node – if there is no service on the current node, it will proxy requests to a node with the service.

Note that this works only in mad-hatter.

I have made the change for the query workbench, and it appears to work correctly. Note that you need to rebuild, and restart ns_server, to get changes to pluggable-ui-*.json.in picked up.
======