[AIRFLOW-2004] Import flash from flask not flask.login

[bolkedebruin]

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-2004

Description

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

import from right module.

Tests

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

-na-

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
  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"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #2943 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2943      +/-   ##
==========================================
- Coverage   73.47%   73.46%   -0.01%     
==========================================
  Files         160      160              
  Lines       12208    12208              
==========================================
- Hits         8970     8969       -1     
- Misses       3238     3239       +1

Impacted Files	Coverage Δ
airflow/www/views.py	71.71% <100%> (ø)	⬆️
airflow/models.py	86.95% <0%> (-0.05%)	⬇️

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[Fokko]

Thanks Bolke!