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 97.17%. Comparing base (bdecb4c) to head (a402015).
Report is 1 commits behind head on staging.

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #3218      +/-   ##
===========================================
- Coverage    97.35%   97.17%   -0.18%     
===========================================
  Files           59       60       +1     
  Lines        13163    12334     -829     
  Branches       300      308       +8     
===========================================
- Hits         12815    11986     -829     
  Misses         334      334              
  Partials        14       14              

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[lmcnulty]

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