chore: update version and changelog for v0.8.0

[cfm]

Description

Towards freedomofpress/securedrop-workstation#600.

Test Plan

• CI passes.
• 0.8.0 is the agreed-upon version.
• There have been no further additions to main since b1525ffe47048b39a6f55fa25cf84e691bde1603.
• Changelog looks sensible on visual review against https://github.com/freedomofpress/securedrop-client/compare/0.7.0..main.

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 is self-contained and 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]

Adding comment to make it easier to check links in the changelog:

• Drop historical versions of database schema (fix: squash Alembic migrations; eliminate non-self-contained data-migration tests #1517)
• Replace "show/hide passphrase" widget with checkbox to increase accessibility (Replace hide/show Passphrase icon control, with checkbox control #1430)
• Remove enclosing directory when all submissions have been deleted for a source (When all submissions are deleted for a source, delete the enclosing directory #1475)
• Support Debian Bullseye (Rebuilding securedrop-client wheels for Python3.9 #1494, Fix deprecation warnings, comparing int and float values #1511, Update AppArmor profile for bullseye support #1528)
• Support Qubes 4.1 (Unbreak export on Qubes OS R4.1 #1495, Add /dev/xen/hypercall to AppArmor allow-list #1530)
• Allow translations to be enabled/disabled per language (build: package only translations explicitly marked as supported #1497, Translations update from Weblate #1527)
• Handle terminating threads cleanly (Ensure that the export and API threads are closed before the corresponding QThread instances are destroyed #1512, Ensure that the sync and both queue threads are closed before the corresponding QThread instances are destroyed #1518, Ensure neither sync or queue threads are likely to be mis-managed by accident #1519)
• (Dev) Add tooling for visualizing the codebase (docs: add make docs target to generate inheritance/call graphs via Doxygen #1484)
• (Dev) Refactor export and print dialogs for maintainability (Refactor extract conversation.ExportFileDialog #1492, Refactor extract conversation.PrintFileDialog #1498, Refactor export tests for portability #1502, Fix missing and incorrect assertions in Controller tests #1501)
• (Dev) Refactor signals according to Qt best practices (Refactor ensure signals are used as recommended by Qt #1508)
• (Dev) Prevent intermittent tests suite errors (Ensure QApplication is instanciated only once in tests #1515)
• (Dev) Decouple database tests from models (fix: squash Alembic migrations; eliminate non-self-contained data-migration tests #1517)
• (Dev) Support development on Debian Bullseye (Update client dev environment for bullseye #1496)