Refactor: Improve handling activity date

[schbetsy]

Minor improvement to the way we assign date values in the parser.

Changes

• Make activity_date a property of the parser object. A new parser object is created for each M2 file, and the Activity Date value applies to all records in a Metro2 file, so no specificity is lost.
• Add type hints for methods that return date values
• Move code that handles assigning activity date from the parser to the AccountActivity model. This is the logic that checks whether activity_date has a value (it doesn't when the header segment is missing) and assigns DOAI as activity date when it is missing.

Testing

1. python manage.py test

Assigning DOAI as activity date is tested in test_parser.py by the test called test_parse_file_without_header_line.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated
• Reviewers requested with the Reviewers tool ➡️

[willbarton]

Makes sense to me 👍🏻