[web] Remove div.bottomContainer from Splash

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

Here's how it looks:

depends on D3447

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

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

Reviewed By: palys-swm, benschac

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3448