[export] Animate export wizard so that there is user feedback during large file transfers

[rocodes]

Description

Reported in freedomofpress/securedrop-workstation#1103 (comment) :

During large file transfers, there's not much UI feedback to let the user know an export is still in progress. The "continue" button spins, but the wizard doesn't show any other feedback until the export completes.

Right now, the export wizard is designed to minimize clicks/steps, and take the shortest path to export. There are a couple ways that more feedback during export could be provided:

• Always show the "Working...." (export in progress) screen, optionally with more granualar progress reporting as we develop that (easiest way, but adds one more mouseclick/screen for users)
• Add some sort of progress indicator at the bottom of the wizard. It would have to be added (hidden) to every child page of the wizard or to the wizard footer, and display it on whatever the current page is when an export is happening.
• Something else tbd :)

How will this impact SecureDrop users?

The spinner icon on the "Continue" button may not be enough to tell the user an export is in progress

How would this affect the SecureDrop Workstation threat model?

n/a

User Stories

As a user, I want to understand what is happening while I attempt to export a large file