[web] Add option to open empty pending thread

Summary: Introduce new nav params (pendingThread and sourceMessage)

Test Plan: Create a button which calls useOnCLickPendingSidebar, click it and verify that empty page is displayed (app should not crash)

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D709