Backup script does not include custom logo

[eloquence]

Description

The backup script at https://github.com/freedomofpress/securedrop/blob/develop/install_files/ansible-base/roles/backup/files/backup.py currently does not add the custom logo, because it was not updated after #3057 changed the behavior to stop overwriting logo.png with the custom logo.

Steps to Reproduce

(Requires prod or staging instance.)

1. Customize your logo
2. Run ./securedrop-admin backup
3. Inspect backup contents

Expected Behavior

Backup contains custom logo image.

Actual Behavior

Backup only contains original default logo.

[eloquence]

freedomofpress/securedrop-docs#181 adds backup/restore docs for now, but I would suggest that we fix this properly with the next regular release at the latest.