[web] Merge keys in UPDATE_NAV_INFO

Summary:
Instead of overwriting the old NavInfo, now we will merge the old one with the new one.

This lets us avoid having to bind navInfo at every place we call UPDATE_NAV_INFO.

The only place we weren't previously merging the keys was on line 149 of web/app.react.js. To make sure this diff wouldn't change that behavior, I confirmed that the value that is set there – which comes from navInfoFromURL – includes values for all keys of NavInfo.

Test Plan: Flow

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D458