Fixed spinner positioning

[noisecapella]

What are the relevant tickets?

Fixes #465

What's this PR do?

Sets the container div for the loader to use relative positioning so the inner spinner will be centered within that, rather than centered inside the entire page.

How should this be manually tested?

In api.js replace getUserProfile with this:

export function getUserProfile(username) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(mockableFetchJSONWithCSRF(`/api/v0/profiles/${username}/`));
    }, 100000);
  });
}

This will delay the load of the user page for 100 seconds. Go to /users/ and see that the spinner is where it should be. You should be able to resize the browser without affecting the position of the spinner.

Screenshots (if appropriate)

[gsidebo]

do we care about EOF linebreaks? everything looks good here other than that possible comment

[gsidebo]

👍

[alicewriteswrongs]

@gsidebo it seems like we don't in general. I dislike seeing the little 🚫 symbol, but ¯_(ツ)_/¯