Use Poppler for PDF extraction

.gitlab-ci.yml

Dockerfile

arkindex/dataimport/api.py

arkindex/dataimport/management/commands/import.py

arkindex/dataimport/models.py

arkindex/dataimport/serializers.py

arkindex/dataimport/tasks/pdf.py

arkindex/dataimport/tests/test_imports.py

arkindex/dataimport/tests/test_pdf.py

arkindex/project/serializer_fields.py

base/Dockerfile

requirements.txt