Added spinner for dashboard

[noisecapella]

What are the relevant tickets?

Fixes #587

What's this PR do?

Adds a spinner to the dashboard page when it is loading, and moved some dashboard tests from App_test.js into new DashboardPage_test.js

Where should the reviewer start?

How should this be manually tested?

View the dashboard over a slow connection. To simulate a slow connection replace the function in api.js with this:

export function getDashboard() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(mockableFetchJSONWithCSRF('/api/v0/dashboard/', {}, true));
    }, 3000);
  });
}

Screenshots (if appropriate)

[giocalitri]

empty line at the end of the file

[giocalitri]

small comment but looks good

[giocalitri]

👍

[pdpinch]

We should get in the habit of consulting with Rob on things like this. I'm going to tag him on the issue.