Unstarring consistently fails on Qubes

[eloquence]

Note: I'm not able to reproduce this issue in the Docker development environment on current master, only in a Qubes environment built from the securedrop-workstation repo as of today. So, this may no longer be valid, but filing in case it's a Qubes-specific issue.

Steps to reproduce:

1. Sign in to a SecureDrop instance that has some submissions.
2. Star a submission.
3. Once it is starred, unstar the submission.

Expected behavior:

Submission is unstarred.

Actual behavior:

Unstarring fails with "Failed to apply change"

[zenmonkeykstop]

Confirmed for me on current master in Qubes.

[redshiftzero]

yep this appears to be Qubes only

[redshiftzero]

note that logs are in ~/.securedrop_client/logs/*.log in sd-svs (fishing out the relevant log lines helps with repro)

[redshiftzero]

since freedomofpress/securedrop#3977 was resolved, this is also resolved on staging instances provisioned on develop