[web] Added LoadingIndicator to LoginForm "Log in" button

Summary:
Added spinning LoadingIndicator to "Log in" button on login screen. More context on Linear.

Video:

Test Plan: Logged out and logged in multiple times on Chrome/Safari and ensured everything worked, covering cases where both login failed and login succeeded.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3605