This is extracted from #8094, where I have run into some snags. Since
these ESLint fixes aren’t actually connected to the Ember 3.16 update
but involve changes to many files, we might as well address them
separately. Where possible I fixed the problems but in cases where
a fix seemed too involved, I added per-line or -file exceptions.
The impact was the application error was no longer being nulled out,
causing the application error to continue to be shown after
transitioning.
This never happened in apps since it's not possible to transition away
from the error screen.
deactivate happens _after_ the new route's model hook, which
results in the possibility of canceling new requests right
after they are made rather than existing open connections
