[usage] Adding loading state indicator when retrieving usage records

[easyCZ]

When retrieving the usage report, we should show a loading indicator while the request is pending a result.

On slow connections, or for large results, the user is presented with a screen showing no Usage available which then gets replaced by the results. This shouldn't be the case as it creates the wrong experience for the user.

Acceptance Criteria:

• When request for a usage report is pending, a loading indicator is shown
• When switching months, a loading indicator is also shown