Improve the openConversationWith test helper

`openConversationWith` currently has a two-step process: a) wait & find elements that are in the user list, b) filter those elements to find the correct one.

This is poor because:

* The item we require might not be in the user list straight away, and hence we might finish the first step prematurely (not generally an issue at the moment, but possibly later)
* Having a two-step process is unnecessary as css will let us search for exactly what we need.

This may help/fix the [intermittent failure for "element is no longer attached to the DOM"](https://trello.com/c/KPnc8EFd), as it will reduce the scope for additional activities happening between waiting for the elements to appear and clicking on one. However, there are other issues there that I'll document on the card.