[web] Render calendar navigation tab conditionally

Summary: Render calendar tab only if it is enabled - like in mobile app. By default it is enabled and the setting is not persisted after page refresh yet.

Test Plan: After introducing apps page (next diff), run web app and switch the calendar in Apps page. The navigation link to calendar should appear only if calendar enabled.

Reviewers: palys-swm, ashoat, benschac, atul

Reviewed By: palys-swm, ashoat

Subscribers: benschac, ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3284