[web] Introduce navigation for enabled apps tab

Summary:
Extend existing navigation to support new panel for apps management.
App Directory component in introduced in the following diff - it's only temporary mock to confirm that navigation works
Also introduced conditional Calendar tab rendering (as it will be optional app)

The video shows the tab after introducing App Directory component:

Test Plan: Run web and check if navigation to Apps tab works correctly

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3265