[AIRFLOW-1740] Fix xcom creation and update via RBAC UI (master)

[feluelle]

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-1740
  • 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).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

• Here are some details about my PR, including screenshots of any UI changes:

Same as #5560 but targeted master branch

cc @ashb

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

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  • 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

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #5561 into master will decrease coverage by <.01%.
The diff coverage is 58.82%.

@@            Coverage Diff            @@
##           master   #5561      +/-   ##
=========================================
- Coverage      79%     79%   -0.01%     
=========================================
  Files         489     489              
  Lines       30733   30741       +8     
=========================================
+ Hits        24282   24287       +5     
- Misses       6451    6454       +3

Impacted Files	Coverage Δ
airflow/www/views.py	75.15% <42.85%> (-0.16%)	⬇️
airflow/models/xcom.py	80.39% <70%> (+0.19%)	⬆️
airflow/models/taskinstance.py	93.18% <0%> (+0.16%)	⬆️

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