Repeated boilerplate code between upload to database forms

[exemplary-citizen]

Is your feature request related to a problem? Please describe.

class CsvToDatabaseForm(DynamicForm):
    # pylint: disable=E0211
    def csv_allowed_dbs() -> List[Database]:  # type: ignore
        csv_enabled_dbs = (
            db.session.query(Database).filter_by(allow_csv_upload=True).all()
        )
        return [
            csv_enabled_db
            for csv_enabled_db in csv_enabled_dbs
            if CsvToDatabaseForm.at_least_one_schema_is_allowed(csv_enabled_db)
        ]

There's unnecessary duplication here between CSV and other forms.

Describe the solution you'd like
We should abstract this into a class UploadToDatabaseForm(DynamicForm) with all the repeated boilerplate and then extend those into class CsvToDatabaseForm(UploadToDatabaseForm), class ExcelToDatabaseForm(UploadToDatabaseForm) etc.

cc: @villebro