Use async lettre transports for sending emails

[Turbo87]

At the moment we are using synchronous lettre transports for sending emails, which means that emails.send(...) needs to be wrapped in a spawn_blocking() call to not block the async tokio worker threads.

This PR migrates our Emails struct to use async transports underneath. It now offers an async_send() method, and (for now) a send() method that uses Handle::block_on() to be able to still send emails from blocking worker threads.

Unfortunately, some of our test code is currently still mixing sync and async code in interesting ways, which meant that some tests had to be slightly adjusted with spawn_blocking() calls to make them happy with the new implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 93.41317% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.17%. Comparing base (64c1aa7) to head (410afc9).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/email.rs	81.25%	6 Missing ⚠️
src/worker/jobs/sync_admins.rs	57.14%	3 Missing ⚠️
src/controllers/user/resend.rs	92.85%	1 Missing ⚠️
src/tests/util/test_app.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9961      +/-   ##
==========================================
+ Coverage   89.16%   89.17%   +0.01%     
==========================================
  Files         292      292              
  Lines       30316    30380      +64     
==========================================
+ Hits        27031    27092      +61     
- Misses       3285     3288       +3     

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