nav: Use our named `getNav` for nav middleware's selector.
nav: Use our named `getNav` for nav middleware's selector.

This saves repeating it; and it means we have a type annotation.

(Inspired by @borisyankov's changes in #2719.)

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile