refactor/login: Refactor heavily-entangled LoginForm component

[liangyuanruo]

Problem

The login page and its components were heavily entwined. This made it difficult to reason about interactions and dependencies, or to remove its Redux dependenices.

Part of #955

Solution

Refactor the LoginForm component to be fully functional. Instead of having inside knowledge of the login flow, it is now configured from outside the component. Instead of hardcoding if-else statements to decide which components to show, it now accepts children as props.

It has been upgraded to become a fully controlled component via the value prop, whereas previously it was relying on a hack using the hidden property to remove DOM input (i.e. user email) when the login page transits from email stage to the OTP stage.

[yong-jie]

lgtm!