refactor: move SupersetAppInitializer to specific initialization package

[ofekisr]

SUMMARY

move SupersetAppInitializer from app.py to initialization package to achieve separation of concerns

[codecov]

Codecov Report

Merging #15278 (b70407c) into master (b1689aa) will decrease coverage by 0.22%.
The diff coverage is 88.38%.

@@            Coverage Diff             @@
##           master   #15278      +/-   ##
==========================================
- Coverage   77.24%   77.01%   -0.23%     
==========================================
  Files         972      973       +1     
  Lines       50366    50371       +5     
  Branches     6140     6140              
==========================================
- Hits        38905    38794     -111     
- Misses      11257    11373     +116     
  Partials      204      204              

Flag	Coverage Δ
hive	?
mysql	81.66% <88.38%> (+<0.01%)	⬆️
postgres	81.68% <88.38%> (+<0.01%)	⬆️
presto	?
python	81.77% <88.38%> (-0.44%)	⬇️
sqlite	81.31% <88.38%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/initialization/__init__.py	88.30% <88.30%> (ø)
superset/app.py	81.25% <100.00%> (-6.44%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.20% <0.00%> (-17.21%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-6.53%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	88.22% <0.00%> (-1.65%)	⬇️
superset/db_engine_specs/base.py	87.97% <0.00%> (-0.41%)	⬇️
superset/models/core.py	89.76% <0.00%> (-0.27%)	⬇️
superset/utils/core.py	88.93% <0.00%> (-0.13%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b1689aa...b70407c. Read the comment docs.

[amitmiran137]

Clear separation between the create_app function to the ApplicationInitiator class