[web] Move bulk of LoginModal to LoginForm

Summary:
We want to pull a LoginForm component out of LoginModal so it can be re-used in Splash.

Next diffs:

1. Naively add LoginForm to Splash
2. 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:

1. yarn dev in web and server (to avoid any hot reloading artifacts).
2. Go to localhost/comm/.
3. Click "Log in".
4. Click the X in the top-right corner to make sure the modal disappears.
5. Enter invalid credentials and make sure error message + input focus is correct.
6. Enter valid credentials and make sure we are correctly "redirected" to the app.

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

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3491