[#2418] Add employee mail-templates

[jiromaykin]

issue https://taiga.maykinmedia.nl/project/open-inwoner/task/2418

Will look better in reality when there is actual content...
The nested table cells grow depending on how much content they contain.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (475ae94) to head (7e87a2e).

❗ Current head 7e87a2e differs from pull request most recent head 0ec9772. Consider uploading reports for the commit 0ec9772 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1187   +/-   ##
========================================
  Coverage    95.18%   95.18%           
========================================
  Files          956      956           
  Lines        34603    34603           
========================================
  Hits         32937    32937           
  Misses        1666     1666           

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