Fix Django deprecations for upgrade path to 5.2

[viniciusandrade]

Summary

Phase 2 of the Django 2.2 → 5.2 upgrade plan. Fixes all known deprecations and removes abandoned dependencies while still on Django 2.2 — all changes are backward-compatible.

• Replace ugettext_lazy/ugettext with gettext_lazy/gettext across 72 files
• Remove django.utils.six, force_text, and unused smart_text imports
• Replace abandoned django-form-utils with minimal utils/betterforms.py shim
• Remove dead recaptcha-client dependency
• Remove deprecated default_app_config from utils/__init__.py
• Remove form_utils from INSTALLED_APPS

Test plan

• Full test suite passes (200 tests across 13 apps)
• No remaining ugettext, force_text, django.utils.six, or form_utils references
• New tests for WsDecsSerializer (8 tests) and betterforms shim (9 tests)
• Verify biblioref form fieldsets render correctly in browser
• Verify suggest reCAPTCHA still works without recaptcha-client package