[web] Center LoginForm vertically/horizontally in Splash component

Summary:
Center LoginForm in Splash so it's positioned properly on the page.

Also copied user-settings-modal.css, which was being used by log-in-form.react.js, into log-in-form.css. The files are identical at this point to start off in a place where things are consistent.

Next diffs:

1. Remove styles that are obviously irrelevant for the LoginForm
2. Style the LoginForm so it looks good in Splash component

Test Plan:
Here's how it looks:

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

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3501