[web] Remove everything from Splash except Log in button

Summary:
Remove everything from Splash except for the "Log in" button.

(Added some placeholder styles that will be shortly removed)

depends on D3449

Test Plan: Same as D3447 (tldr: make sure we can still log in, etc)

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

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3450