[web] Add LoginForm to Splash

Summary:
Add LoginForm to Splash so users can log in directly without having to open a modal first.

Next diffs:

1. Style LoginForm so it looks decent in both Splash and LoginModal. Currently the LoginModal/LoginForm is styled using user-settings-modal.css. Will create a separate stylesheet for LoginForm but will cut/paste the styles to start with something consistent.

Test Plan:
Here's how it looks (for now):

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

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3492