First pass at moving 'Mijn Berichten' into a CMS apphook

[Bartvaderkin]

Same as the other first-passes:

I kept the views and tests at their current sub-optimal locations and basically moved the urls.

Most of the changes are because this changed the url namespace.

[codecov-commenter]

Codecov Report

Merging #584 (aaebae0) into feature/django-cms (398c808) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           feature/django-cms     #584      +/-   ##
======================================================
- Coverage               96.71%   96.70%   -0.02%     
======================================================
  Files                     552      555       +3     
  Lines                   19481    19510      +29     
======================================================
+ Hits                    18842    18867      +25     
- Misses                    639      643       +4     

Impacted Files	Coverage Δ
src/open_inwoner/accounts/tests/test_auth.py	100.00% <ø> (ø)
src/open_inwoner/accounts/urls.py	100.00% <ø> (ø)
src/open_inwoner/accounts/views/__init__.py	100.00% <ø> (ø)
.../open_inwoner/components/templatetags/link_tags.py	92.72% <ø> (ø)
src/open_inwoner/conf/base.py	95.37% <ø> (ø)
...ounts/management/commands/notify_about_messages.py	100.00% <100.00%> (ø)
src/open_inwoner/accounts/models.py	97.91% <100.00%> (ø)
src/open_inwoner/accounts/tests/test_admin.py	100.00% <100.00%> (ø)
.../open_inwoner/accounts/tests/test_contact_views.py	100.00% <100.00%> (ø)
...rc/open_inwoner/accounts/tests/test_file_upload.py	100.00% <100.00%> (ø)
... and 9 more

... and 1 file with indirect coverage changes

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