fix: examples remove app context at the module level

[dpgaspar]

SUMMARY

Remove bad patterns from the load examples, application context operations at the module level.
Configuration, database operations or other app context operations should execute/evaluate when necessary, while executing a cli operation, a web request a celery task etc, not at module load.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15546 (1dc6be5) into master (15796ea) will decrease coverage by 0.09%.
The diff coverage is 35.00%.

@@            Coverage Diff             @@
##           master   #15546      +/-   ##
==========================================
- Coverage   76.96%   76.86%   -0.10%     
==========================================
  Files         976      976              
  Lines       51305    51319      +14     
  Branches     6912     6912              
==========================================
- Hits        39485    39445      -40     
- Misses      11601    11655      +54     
  Partials      219      219              

Flag	Coverage Δ
hive	81.28% <35.00%> (-0.04%)	⬇️
mysql	81.55% <35.00%> (-0.04%)	⬇️
postgres	81.57% <35.00%> (-0.04%)	⬇️
presto	?
python	81.95% <35.00%> (-0.20%)	⬇️
sqlite	81.18% <35.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/examples/country_map.py	25.64% <0.00%> (-0.68%)	⬇️
superset/examples/long_lat.py	24.39% <0.00%> (-0.61%)	⬇️
superset/examples/world_bank.py	30.55% <20.00%> (-0.44%)	⬇️
superset/examples/bart_lines.py	28.57% <25.00%> (-1.06%)	⬇️
superset/examples/energy.py	26.82% <25.00%> (-0.68%)	⬇️
superset/examples/flights.py	18.18% <25.00%> (-0.57%)	⬇️
superset/examples/multiformat_time_series.py	17.02% <25.00%> (-0.38%)	⬇️
superset/examples/paris.py	28.00% <25.00%> (-1.17%)	⬇️
superset/examples/sf_population_polygons.py	28.00% <25.00%> (-1.17%)	⬇️
superset/examples/deck.py	12.50% <33.33%> (-1.35%)	⬇️
... and 6 more

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[villebro]

Much better, thanks for cleaning this up!