feat: CRUD REST API for CSS Templates

[dpgaspar]

SUMMARY

New CRUD REST API for css templates ( api/v1/css_template )

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-commenter]

Codecov Report

Merging #11114 into master will decrease coverage by 4.51%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master   #11114      +/-   ##
==========================================
- Coverage   65.75%   61.23%   -4.52%     
==========================================
  Files         818      824       +6     
  Lines       38783    38895     +112     
  Branches     3655     3655              
==========================================
- Hits        25501    23817    -1684     
- Misses      13173    14898    +1725     
- Partials      109      180      +71     

Flag	Coverage Δ
#cypress	?
#javascript	62.24% <ø> (ø)
#python	60.63% <91.30%> (-0.55%)	⬇️

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

Impacted Files	Coverage Δ
superset/css_templates/dao.py	80.95% <80.95%> (ø)
superset/css_templates/commands/bulk_delete.py	88.46% <88.46%> (ø)
superset/css_templates/filters.py	92.85% <92.85%> (ø)
superset/css_templates/api.py	95.45% <95.45%> (ø)
superset/app.py	80.45% <100.00%> (+0.15%)	⬆️
superset/css_templates/commands/exceptions.py	100.00% <100.00%> (ø)
superset/css_templates/schemas.py	100.00% <100.00%> (ø)
superset-frontend/src/SqlLab/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/explore/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/dashboard/App.jsx	0.00% <0.00%> (-100.00%)	⬇️
... and 187 more

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