Replace sdw-login.py and .desktop file with systemd user unit

[rocodes]

Status

Ready for review [will flip out of draft mode after testing]

Description of Changes

Fixes #1073

Changes proposed in this pull request:

• Add user systemd unit to autostart sdw-updater (updater/login)
• remove sdw-login.py and autostart .desktop file

Testing

• CI passing
• Manual testing: login/updater appear on start of user session

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing pilot instances
2. New installs

Checklist

If you have made changes to the provisioning logic

• All tests (make test) pass in dom0

If you have added or removed files

• I have updated MANIFEST.in and rpm-build/SPECS/securedrop-workstation-dom0-config.spec

If documentation is required

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation

[rocodes]

Commenting based on discussion: we may want to revisit whether autostart is a feature we want to support in any case

[rocodes]

(Once #1088 lands I will update this PR to use the same approach and add this unit to the presets file)