fix(dashboard): commit update once

[serenajiang]

SUMMARY

Recent changes to dashboard updates caused incorrect partial updates to be committed, resulting in malformed json in some db engines (mysql) and breaking dashboards that contained non ascii characters in headers or custom chart titles. This PR ensures that we only commit when all updates are complete.

This issue emerged in #17570. I verified that #17766 seemed to fix the problem in a test env, but it turns out this problem is likely db specific, so it did not actually address the root cause.

The root cause:

• DashboardDAO.update updates all properties naively, including json_metadata, which includes positions
• DashboardDAO.update_charts_owners commits the changes done in DashboardDAO.update. Something goes wrong when committing a string with emojis, resulting in a truncated dashboard.json_metadata for mysql.
• DashboardDAO.set_dash_metadata fetches dashboard.params_dict, which fails to parse due to malformed json. Error is thrown.

Other locations where we save json with emojis inadvertently use json.dumps(json.loads(...)) to encode the emojis in a mysql-friendly way, so we didn't see this before.

TESTING INSTRUCTIONS

Verified change locally with mysql db.

ADDITIONAL INFORMATION

@geido @kgabryje @graceguo-supercat @etr2460

[codecov]

Codecov Report

Merging #17781 (4746c99) into master (c18d486) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17781      +/-   ##
==========================================
- Coverage   67.78%   67.72%   -0.07%     
==========================================
  Files        1605     1605              
  Lines       64200    64208       +8     
  Branches     6790     6790              
==========================================
- Hits        43519    43482      -37     
- Misses      18825    18870      +45     
  Partials     1856     1856              

Flag	Coverage Δ
hive	81.79% <100.00%> (+<0.01%)	⬆️
mysql	82.17% <100.00%> (+<0.01%)	⬆️
postgres	82.22% <100.00%> (+<0.01%)	⬆️
presto	?
python	82.56% <100.00%> (-0.15%)	⬇️
sqlite	81.90% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/dashboards/commands/update.py	84.28% <100.00%> (+0.46%)	⬆️
superset/db_engine_specs/presto.py	84.34% <0.00%> (-6.06%)	⬇️
superset/connectors/sqla/models.py	87.07% <0.00%> (-1.35%)	⬇️
superset/dashboards/dao.py	95.55% <0.00%> (-0.75%)	⬇️
superset/models/core.py	89.26% <0.00%> (-0.74%)	⬇️
superset/databases/schemas.py	98.47% <0.00%> (+0.02%)	⬆️
superset/reports/commands/alert.py	96.51% <0.00%> (+0.04%)	⬆️

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[geido]

LGTM. Thanks for the fix!