[AIRFLOW-1755] Allow mount below root

[bolkedebruin]

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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-1755

Description

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

This enables Airflow and Celery Flower to live
below root. Draws on the work of Geatan Semet
(@stibbons).

This closes #2723 and closes #2818

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
  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

cc: @stibbons @artwr @ashb @jgao54 @Fokko

If we find the need to mount the API separately, we just have to put it in a separate "app" and mount it in the dispatcher.

[ashb]

No comments from me -- code looks simple, but I don't have time right now to test we've got all the URLs in the templates updated everywhere.

[gsemet]

+1

[codecov-io]

Codecov Report

Merging #2952 into master will increase coverage by 0.1%.
The diff coverage is 63.15%.

@@            Coverage Diff            @@
##           master    #2952     +/-   ##
=========================================
+ Coverage   73.49%   73.59%   +0.1%     
=========================================
  Files         161      161             
  Lines       12237    12251     +14     
=========================================
+ Hits         8993     9016     +23     
+ Misses       3244     3235      -9

Impacted Files	Coverage Δ
airflow/bin/cli.py	54.68% <0%> (-0.26%)	⬇️
airflow/www/views.py	71.68% <50%> (-0.04%)	⬇️
airflow/www/app.py	98.95% <91.66%> (+2.4%)	⬆️
airflow/jobs.py	79.92% <0%> (+0.44%)	⬆️
airflow/utils/helpers.py	54.02% <0%> (+2.87%)	⬆️
airflow/task/task_runner/bash_task_runner.py	100% <0%> (+6.66%)	⬆️

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