add mailer for collection invitations

[kkoehn]

fixes #1413

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.77%. Comparing base (44a417a) to head (d49dc83).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2057      +/-   ##
==========================================
+ Coverage   94.76%   94.77%   +0.01%     
==========================================
  Files         134      135       +1     
  Lines        3421     3429       +8     
==========================================
+ Hits         3242     3250       +8     
  Misses        179      179              

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