(This change could be considered a little risky so we should keep an eye
on it.)
The core issue here is that sometimes as far as the router was concerned
we had transitioned to a route even if a Topic model was still in the
process of loading. In this case the callback could not retrieve the
correct title yet because it had not loaded.
This fix returns a promise from `setupController` when visiting a topic,
which will have the router block until the topic is loaded. This means
that the transition never triggers until the topic title is present.
Note: adding a test for this is basically impossible - it was super hard
to reproduce even in a browser.
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