feat(alert&report): set image width for email

[lilykuang]

SUMMARY

When sending a report of a dashboard or a chart, the image in the email is very wide and requires users to scroll to the right to the see the entire report.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

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 #14527 (0af0e91) into master (d1d98d8) will decrease coverage by 0.24%.
The diff coverage is n/a.

❗ Current head 0af0e91 differs from pull request most recent head 34470ea. Consider uploading reports for the commit 34470ea to get more accurate results

@@            Coverage Diff             @@
##           master   #14527      +/-   ##
==========================================
- Coverage   77.32%   77.08%   -0.25%     
==========================================
  Files         958      958              
  Lines       48299    48299              
  Branches     5658     5658              
==========================================
- Hits        37346    37229     -117     
- Misses      10751    10868     +117     
  Partials      202      202              

Flag	Coverage Δ
hive	?
mysql	81.06% <ø> (ø)
postgres	81.10% <ø> (ø)
presto	?
python	81.18% <ø> (-0.46%)	⬇️
sqlite	80.70% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/reports/notifications/email.py	100.00% <ø> (ø)
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	70.32% <0.00%> (-17.08%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-6.95%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	88.37% <0.00%> (-1.70%)	⬇️
superset/db_engine_specs/base.py	87.66% <0.00%> (-0.44%)	⬇️
superset/models/core.py	88.85% <0.00%> (-0.28%)	⬇️
superset/utils/core.py	88.87% <0.00%> (-0.13%)	⬇️

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