Only render things once - fixes #3103

[benfrancis]

This fixes #3103 by waiting for both the /things and /groups WebSocket connections to open before firing a refresh things event, rather than firing it twice.

I don't love this because it will slightly delay the rendering of Things on the screen until two TCP sockets have opened, but otherwise we're doing all the work twice.

There is still room for improvement because the /things endpoint is still fetched twice (it was fetched three times before), and I think in the thing detail view refreshThing() is still called a second time once the sockets have opened.

Further refinements could potentially be made by implementing the W3C WoT Discovery Directory Service API (#2857), which provides more information in event payloads so that the whole collection of things doesn't have to be re-rendered for every small change to a Thing.

[tim-hellhake]

If I'm not mistaken this will also change the behavior for reconnects.
Before refreshThings() was called for every connect event, now it's only called for the first connect event.
Is this intentional?

[benfrancis]

Hmm, good point. No that wasn't intentional. Might need a re-think...