fix(alerts, thumbnails): celery tasks using flask-sqlalchemy sessions

[dpgaspar]

SUMMARY

celery tasks should use a scoped session with nullpool that closes and removes the session at the end of the context. When using security manager find_user we are using the normal flask-sqlalchemy session

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #11950 (95e3e84) into master (a7bba92) will decrease coverage by 9.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11950      +/-   ##
==========================================
- Coverage   64.45%   55.10%   -9.36%     
==========================================
  Files         937      425     -512     
  Lines       45319    14871   -30448     
  Branches     4317     3825     -492     
==========================================
- Hits        29211     8194   -21017     
+ Misses      15948     6677    -9271     
+ Partials      160        0     -160     

Flag	Coverage Δ
cypress	55.10% <ø> (+27.22%)	⬆️
javascript	?
python	?

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

Impacted Files	Coverage Δ
...uperset-frontend/src/dashboard/util/dnd-reorder.js	0.00% <0.00%> (-100.00%)	⬇️
...rset-frontend/src/dashboard/util/getEmptyLayout.js	0.00% <0.00%> (-100.00%)	⬇️
...dashboard/components/resizable/ResizableHandle.jsx	0.00% <0.00%> (-100.00%)	⬇️
.../src/dashboard/util/getFilterScopeFromNodesTree.js	0.00% <0.00%> (-93.48%)	⬇️
...src/dashboard/components/gridComponents/Header.jsx	10.52% <0.00%> (-86.85%)	⬇️
...rc/dashboard/components/gridComponents/Divider.jsx	13.33% <0.00%> (-86.67%)	⬇️
...ontend/src/dashboard/util/getDashboardFilterKey.ts	14.28% <0.00%> (-85.72%)	⬇️
...nd/src/views/CRUD/data/query/QueryPreviewModal.tsx	14.70% <0.00%> (-82.97%)	⬇️
...set-frontend/src/views/CRUD/welcome/EmptyState.tsx	5.71% <0.00%> (-82.10%)	⬇️
...et-frontend/src/SqlLab/components/TableElement.jsx	4.70% <0.00%> (-81.35%)	⬇️
... and 834 more

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[villebro]

LTGM. I wonder if we should introduce some structures that explicitly require scoped sessions and nullpools when called from celery.