Use `deliver_later` to send emails

[MrSerth]

Now, we are only using deliver_now for login-based emails, such as password reset or confirmation emails.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.52%. Comparing base (d99a715) to head (f841f5a). Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
app/controllers/comments_controller.rb	0.00%	2 Missing :warning:
app/controllers/request_for_comments_controller.rb	0.00%	1 Missing :warning:
lib/tasks/detect_exercise_anomalies.rake	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2581 +/- ## ======================================= Coverage 69.52% 69.52% ======================================= Files 202 202 Lines 6360 6360 ======================================= Hits 4422 4422 Misses 1938 1938 ```

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