[AIRFLOW-6734] Use configured base_template instead of hard-coding

[ashb]

Flask/FAB gives us a base_template variable that we should use instead
of hard-coding a specific template to extend.

At the time this PR was submitted, airflow/master.html is empty apart from a call to extend the customized appbuilder/baselayout.html, which is the current value of base_template. That is tidied up in AIRFLOW-6733/#7366, but this change has been done separately as they can be merged in either order without any ill-effect.

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Issue link: AIRFLOW-6734

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[codecov-io]

Codecov Report

Merging #7367 into master will increase coverage by 53.33%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #7367       +/-   ##
===========================================
+ Coverage   32.84%   86.17%   +53.33%     
===========================================
  Files         871      871               
  Lines       40627    40627               
===========================================
+ Hits        13343    35011    +21668     
+ Misses      27284     5616    -21668

Impacted Files	Coverage Δ
airflow/bin/airflow	84.61% <0%> (ø)	⬆️
airflow/plugins_manager.py	86.95% <0%> (+0.72%)	⬆️
airflow/bin/cli.py	94.73% <0%> (+2.1%)	⬆️
airflow/executors/celery_executor.py	88.51% <0%> (+3.37%)	⬆️
airflow/exceptions.py	100% <0%> (+3.57%)	⬆️
airflow/utils/decorators.py	90.47% <0%> (+4.76%)	⬆️
airflow/kubernetes/pod_launcher.py	92.25% <0%> (+5.63%)	⬆️
airflow/www/forms.py	100% <0%> (+8%)	⬆️
airflow/utils/timeout.py	75% <0%> (+8.33%)	⬆️
airflow/kubernetes/kube_client.py	85.71% <0%> (+8.57%)	⬆️
... and 731 more

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