[AIRFLOW-3233] Fix deletion of DAGs in the UI

[aoen]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Currently deleting DAGs that exist on the scheduler but not the webserver is not possible because the DAG id in the deletion URL comes from the webserver DAG object.
  e.g. http://localhost:8080/delete?dag_id= instead of http://localhost:8080/delete?dag_id=example_kubernetes_executor

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  UI Fix, tested both RBAC and regular.

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4069 into master will decrease coverage by 0.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4069      +/-   ##
==========================================
- Coverage   76.67%   75.91%   -0.76%     
==========================================
  Files         199      199              
  Lines       16186    15961     -225     
==========================================
- Hits        12410    12117     -293     
- Misses       3776     3844      +68

Impacted Files	Coverage Δ
airflow/operators/docker_operator.py	0% <0%> (-97.65%)	⬇️
airflow/operators/slack_operator.py	0% <0%> (-97.37%)	⬇️
airflow/operators/s3_to_hive_operator.py	0% <0%> (-94.02%)	⬇️
airflow/security/kerberos.py	0% <0%> (-71.43%)	⬇️
airflow/operators/latest_only_operator.py	25% <0%> (-65%)	⬇️
airflow/operators/subdag_operator.py	70.96% <0%> (-19.36%)	⬇️
airflow/operators/python_operator.py	81.98% <0%> (-13.05%)	⬇️
airflow/utils/file.py	76% <0%> (-8%)	⬇️
airflow/operators/hive_operator.py	80.76% <0%> (-5.77%)	⬇️
airflow/executors/dask_executor.py	0% <0%> (-2%)	⬇️
... and 27 more

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[ashb]

Is it possible to add unit tests for this?

What is the difference in value of dag_id vs dag.dag_id? Which view was failing?

[ashb]

Minor nit: this includes the comment in thee HTML which we don't need. If you think the comment is useful then:

Suggested change

	<!-- Use dag_id instead of dag.dag_id, because the DAG might not exist in the webserver's DagBag -->
	{# Use dag_id instead of dag.dag_id, because the DAG might not exist in the webserver's DagBag #}

[ashb]

Should be possible to test this by creating a Dag object in the DB and ensuring the correct delete link appears in the output