Update email template for mailersend with clean HTML

[lmcnulty]

Supercedes #3099.

Light mode:

Also adds a script to test the template without running the whole site at site/gatsby-site/src/scripts/sendEmailTest.js.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.43%. Comparing base (e6490c6) to head (281cc72).
Report is 15 commits behind head on staging.

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #3218      +/-   ##
===========================================
- Coverage    94.81%   94.43%   -0.38%     
===========================================
  Files          111      113       +2     
  Lines        13528    12720     -808     
  Branches       696      706      +10     
===========================================
- Hits         12826    12012     -814     
- Misses         674      688      +14     
+ Partials        28       20       -8     

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[kepae]

This looks great. Please confirm with @cesarvarela if it would be better to merge now or after some of the changes he's making to the notifications logic.

[cesarvarela]

Looks like it is safe to merge it $now

[lmcnulty]

Now updates all the email templates (except the user management ones).