Simplify ProcessedApplicationData with mixins and examine adapter for resume validation

[waalbert]

Continuing from #483 , because of common attributes between each type of application such as submission_time and reviews, these shared attributes can be put into a mixin/parent class.

Because #468 added an adapter in user.py to satisfy mypy, let's also take a look at why an adapter is needed or not needed.

try:
    resume_url = await resume_handler.upload_resume(
        # TODO: reexamine why adapter is needed
        TypeAdapter(
            Union[RawHackerApplicationData, RawMentorApplicationData]
        ).validate_python(raw_application_data),
        resume,
    )