Onboarding email formatting

[cstns]

Description

• Adding the concept of email layouts
• starting with a default template that mimics inbound marketing templates
• outbound emails sent by the FF app will use these layouts

Related Issue(s)

closes #4039

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.75%. Comparing base (f8ced67) to head (ae355bd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4110   +/-   ##
=======================================
  Coverage   78.75%   78.75%           
=======================================
  Files         285      286    +1     
  Lines       13045    13048    +3     
  Branches     2907     2907           
=======================================
+ Hits        10273    10276    +3     
  Misses       2772     2772           

Flag	Coverage Δ
backend	78.75% <100.00%> (+<0.01%)	⬆️

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