includes compiled translation catalogs in Python source distribution

[cfm]

Description

Draft pending #1282 #1302 #1317

Closes #1283 by overriding configuring python setup.py sdist to run and include the outputs of make compile-translation-catalogs gettext portable and machine catalogs produced by Weblate after #1348.

Depends for merge on:

1. mechanizes continuous-localization workflow #1282
2. adds Babel to build requirements securedrop-builder#270
3. Rebase out eaf7142
4. SecureDrop Client localization workflow follow-up #1302
5. provides tooling for interactive localization updates #1348

Test Plan

1. Build a source distribution (python setup.py sdist) and Debian package (per freedomofpress/securedrop-debian-packaging) in this directory. Confirm that .po and .mo files are listed in the output of both build processes.
2. Install the Debian package (e.g., in a disposable VM) and run LANG=es_ES securedrop-client.
3. Observe that the Client's UI is translated.

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

[cfm]

Based on end-to-end testing in #1283, these changes are no longer required to package the gettext portable and machine objects produced by Weblate after #1348.