Currently to access dag in Webserver we use global variable dagbag from airflow.www.views. This global object is also necessary for new Connexion API (/tasks endpoint for example).

However, it would be better to avoid any coupling between views code and API code. This PR proposes to replace this global object with an attribute of Flask app that can be accessed using flask.current_app as follows:

from flask import current_app

@expose('/my_endpoint')
def my_endpoint():
    dag_bag: DagBag = current_app.dag_bag
    ...

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• Description above provides context of the change
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• Target Github ISSUE in description if exists
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