[AIRFLOW-5634] Don't allow disabled fields to be edited in DagModelView

[ashb]

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Jira

• https://issues.apache.org/jira/browse/AIRFLOW-5634

Description

• If someone plays silly games and edits the HTML (or crafts a request) to
  the DAG model it is possible to edit fields that aren't meant to be
  edited, leading to odd to debug behaviour

  Against v1-10-stable as this only applies to the classic UI. Separate PR coming for RBAC UI (where I will just disable the ability to edit the DagModel. I could do the same here )

Tests

• Tests added

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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6307 into v1-10-stable will decrease coverage by <.01%.
The diff coverage is 88.88%.

@@               Coverage Diff                @@
##           v1-10-stable    #6307      +/-   ##
================================================
- Coverage         76.67%   76.66%   -0.01%     
================================================
  Files               509      509              
  Lines             34381    34389       +8     
================================================
+ Hits              26362    26366       +4     
- Misses             8019     8023       +4

Impacted Files	Coverage Δ
airflow/www/views.py	45.1% <88.88%> (+0.31%)	⬆️
airflow/jobs/local_task_job.py	85.71% <0%> (-4.77%)	⬇️
airflow/utils/dag_processing.py	57.14% <0%> (-0.18%)	⬇️

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