[web] Keep sign in button the same size when loading

Summary:
Noticed that the button shifted in height when loading. Prior to this diff, the button was increasing in height when loading, due to an explicit min-height property that gets set when it is disabled. It looks like this was built by an intern several years ago, and the min-height was probably added to make sure the button height didn't go to 0 or some small number.

This diff changes it so the height is based on the text, and the spinner is absolutely-positioned over the text. To keep the height the same even when the text is hidden, we use visibility: hidden for the text instead of display: none.

Depends on D12085

Test Plan:

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12086