User storage not properly reported in the frontend

[rtibbles]

Observed behavior

Usage data is not being properly reported in the frontend: https://community.learningequality.org/t/bug-data-usage-not-showing/2174/2

Leading to people being allowed to upload, but then have unhandled 418 errors when trying to generate a file upload URL.

Expected behavior

The frontend should properly load and display the remaining storage.

We also probably shouldn't use 418 to indicate an insufficient storage error.

User-facing consequences

Not realizing why they can't upload content.

[bjester]

Ran into another upload issue with confusion about 418 status. Definitely recommend swapping that status code

[vkWeb]

@rtibbles I think I can look into this after tags PR...?