[web] Turn Splash into a functional component

Summary:
This should be a noop.

(This component will be renamed from ConnectedSplash to Splash in the next diff).

depends on D3450

Test Plan: Careful reading, flow, and making sure we can still log into the web app, etc.

Reviewers: palys-swm, def-au1t, benschac, varun

Reviewed By: palys-swm, benschac

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3451