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**Description**

Update source_code field of dag_code table to MEDIUMTEXT
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**Use case / motivation**

Lot of dags exceed the limit of 65K characters limit giving error `"Data too long for column 'source_code' at row 1"` when enabling webserver to fetch dag_code from db. 
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**Related Issues**

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airflow/models/dagcode.py
with open_maybe_zipped(fileloc, 'r') as f:
            code = f.read()
        return code
    @classmethod
    @provide_session
    def _get_code_from_db(cls, fileloc, session=None):
        dag_code = session.query(cls).filter(cls.fileloc_hash == cls.dag_fileloc_hash(fileloc)).first()
        if not dag_code:
            raise DagCodeNotFound()
        else:
            code = dag_code.source_code
        return code
    @staticmethod
    def dag_fileloc_hash(full_filepath: str) -> int:
        """Hashing file location for indexing.
        :param full_filepath: full filepath of DAG file
        :return: hashed full_filepath
        """
        
        
        import hashlib
        
        return struct.unpack('>Q', hashlib.sha1(full_filepath.encode('utf-8')).digest()[-8:])[0] >> 8