Fix app context leak in tests

Closes #448 (closed)

This MR:

• Ensures a single instance of the Flask app is used throughout the entire test session
• Ensures the app context is pushed and popped properly throughout the tests
• Ensures the data directory and in-memory database is cleaned in between tests or between modules
• Cleans up some obsolete fixtures that were defined in each module
• Removes unneeded pushes of the app context
• Adds the app context for tests/test_pdf_generation.py::test_generate_datamatrix