[#1530] Confirmation email with CC on cosign

[SilviaAmAm]

Fixes #1530

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (64ffc8b) 94.49% compared to head (fceba71) 94.50%.

❗ Current head fceba71 differs from pull request most recent head c906656. Consider uploading reports for the commit c906656 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3157   +/-   ##
=======================================
  Coverage   94.49%   94.50%           
=======================================
  Files         609      611    +2     
  Lines       19097    19120   +23     
  Branches     2178     2180    +2     
=======================================
+ Hits        18046    18069   +23     
  Misses        759      759           
  Partials      292      292           

Impacted Files	Coverage Δ
src/openforms/config/models.py	96.85% <ø> (ø)
src/openforms/emails/confirmation_emails.py	100.00% <ø> (ø)
src/openforms/emails/models.py	90.90% <ø> (ø)
src/openforms/emails/utils.py	100.00% <ø> (ø)
...rc/openforms/template/backends/sandboxed_django.py	100.00% <ø> (ø)
...penforms/emails/templatetags/cosign_information.py	100.00% <100.00%> (ø)
src/openforms/submissions/tasks/co_sign.py	100.00% <100.00%> (ø)
src/openforms/submissions/tasks/emails.py	100.00% <100.00%> (ø)
src/openforms/submissions/utils.py	93.06% <100.00%> (+0.21%)	⬆️
src/openforms/utils/regex.py	100.00% <100.00%> (ø)

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[SilviaAmAm]

At some point we could do this with one query: django/django#12438

[sergei-maertens]

Some Dutch language remarks left in the docs.