[AIRFLOW-4020] Remove viewer DAG edit permissions

[feng-tao]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-4020
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal(AIP).

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Viewer shouldn't be able to have write/edit access on DAG View-menu

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[ashb]

Why is Viewer not allowed to see this view? It's a read-only view.

[feng-tao]

sorry, wrong unit test. I changed to use /refresh endpoint instead.

[feng-tao]

PTAL @ashb
Tests pass

[codecov-io]

Codecov Report

Merging #4845 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4845      +/-   ##
==========================================
- Coverage   74.73%   74.73%   -0.01%     
==========================================
  Files         449      449              
  Lines       28959    28961       +2     
==========================================
+ Hits        21642    21643       +1     
- Misses       7317     7318       +1

Impacted Files	Coverage Δ
airflow/www/security.py	92.82% <100%> (+0.07%)	⬆️
airflow/contrib/operators/ssh_operator.py	82.27% <0%> (-1.27%)	⬇️

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