[AIRFLOW-6733] Extend not replace template

[ashb]

We had copied whole-sale the baselayout.html from FAB and customized it, but pre-existing mechanism in flask/jinja of template inheritance means we don't have to do this.

We can instead extend the template from FAB and change the few blocks we need to.

I have written this PR as two commits -- one to move the file without any changes (we can't extend appbuilder/baselayout.html from FAB while our file exists, as that is "earlier" in the template search path) and then a second commit that keeps just the blocks we have extended/changed.

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Issue link: AIRFLOW-6733

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[codecov-io]

Codecov Report

Merging #7366 into master will increase coverage by 52.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7366      +/-   ##
==========================================
+ Coverage   32.84%   85.44%   +52.6%     
==========================================
  Files         871      871              
  Lines       40627    40627              
==========================================
+ Hits        13343    34714   +21371     
+ Misses      27284     5913   -21371

Impacted Files	Coverage Δ
airflow/www/app.py	94.24% <ø> (+11.51%)	⬆️
...flow/providers/apache/cassandra/hooks/cassandra.py	21.51% <0%> (-72.16%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/api/auth/backend/kerberos_auth.py	28.16% <0%> (-54.93%)	⬇️
airflow/providers/redis/operators/redis_publish.py	50% <0%> (-50%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/providers/mongo/sensors/mongo.py	53.33% <0%> (-46.67%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0%> (-39.44%)	⬇️
airflow/providers/redis/sensors/redis_key.py	61.53% <0%> (-38.47%)	⬇️
airflow/executors/celery_executor.py	50.67% <0%> (-34.46%)	⬇️
... and 741 more

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[feluelle]

Thanks for the good explanation. I was about to ask why you put this in two commits - now I get it 👍