Backup: bring back the connection screen image

[sergeymitr]

Updating Webpack (#20789) changed the asset loading behavior, and the connection screen image disappeared:

This PR brings it back by reintroducing the asset base URL we previously didn't need.

Changes proposed in this Pull Request:

• Reintroduce the asset base URL.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

1. Disconnect Jetpack.
2. Go to the Backup plugin, and confirm that there's an image on the connection screen:

[github-actions]

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Backup plugin:

• Next scheduled release: September 7, 2021.
• Scheduled code freeze: August 30, 2021.

[samiff]

Tests well for me, thanks for fixing this 👍