Ensure neither sync or queue threads are likely to be mis-managed by accident

[gonzalo-bulnes]

Description

🔮 Reviewers rebase this branch once #1518 is merged, only the last three commits need review 💚

Future-proves #1518 by ensuring that queue job instances don't ever create QThread instances on the fly, that will be destroyed without the underlying thread being closed. Now that we have convenient tools to manage threads (see below) this should be enough to prevent re-introducing un-managed threads in the future.

from securedrop_client.app import threads

with threads(2) as [my_safer_thread, my_second_safer_thread]:
    pass # or do whatever you want with your two QThread instances

# At this point, the QThread instances will be safely destroyed after the underlying threads are closed.

Test Plan

• Confirm that the build is green 🍏
• Optionally, confirm that the sync happens as usual
• Optionally, confirm that the messages are downloaded as usual
• Optionally, confirm that the files can be downloaded as usual

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[sssoleileraaa]

lgtm