[web] Remove div.content from Splash

Summary:
Cut div.content in splash.react.js and associated styles in splash.css.

Here's how it looks:

depends on D3448

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

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

Reviewed By: ashoat, benschac

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3449