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 (c6c9b0b) to head (e3ab947).
⚠️ Report is 23 commits behind head on main.

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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[arkirchner]

I apologize for the late review!

[MrSerth]

Thanks for working on this PR! Would you mind having a quick look at two bugs that are included? First, the mailer preview doesn't work currently (due to the mailer parametrization), and then, the text mail sent isn't correctly formatted.

Suggestion:

CollectionInvitationMailer.with(collection:, recipient: user).send_invitation

[kkoehn]

@MrSerth thanks for noticing the bugs! I created a PR (#2133) to fix these issues