Login/register button should not render until the `/get-current-user/` call has completed

[ml-evs]

We should replace the login button with a spinner until the /get-current-user call to the API has finished, and have better state if it fails.

Similarly, I often see two calls to /get-current-user being made when the page first opens, we should hunt these down and make sure the second one simply waits for the store to be populated with user info. Once stored once, we should probably also use the store directly without making another call to /get-current-user.