[web] First pass at styling LoginForm

Summary:
Started with the existing styles (which were borrowed from user-settings-modal.css) and worked on styling LoginForm so it looks decent.

I don't think it looks good, but it's servicable and there aren't any designs on Figma.

Test Plan:
Here's how it looks:

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

Reviewed By: benschac, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3507