[#2059] Add emailnotification templates

[jiromaykin]

issue: https://taiga.maykinmedia.nl/project/open-inwoner/task/2059
Figma designs: https://www.figma.com/file/iKGhWhstaLIlFSaND2q7cE/OIP---Designs-(new)?type=design&node-id=1080-7543&mode=design&t=6F5ky4GxXhMxiWJ8-0

How to preview e-mail templates:

• go to http://localhost:8000/admin/mail_editor/mailtemplate/

• delete all templates (checkboxes/dropdown)

• in terminal: $ python src/manage.py add_missing_templates

• reload admin page, click "open" to see processed preview, which also contains a field in which you can send yourself a test e-mail (locally: only possible if you have an App Password added to your Gmail Security configurations with 2-factor!)

• might need 'previous case status' def.

Fallbacks may need to be created for border-radius (all Microsoft mail clients unsupported).
Fonts can only be loaded as font-face CDN hosted fonts - is not supported in many email-clients so added fallbacks for commonly used local fonts.

In short: it's going to look quite different in all email-clients and different machines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (79332e4) 94.74% compared to head (cb0008c) 94.79%.
Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #999      +/-   ##
===========================================
+ Coverage    94.74%   94.79%   +0.05%     
===========================================
  Files          872      873       +1     
  Lines        30546    30578      +32     
===========================================
+ Hits         28940    28987      +47     
+ Misses        1606     1591      -15     

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[alextreme]

@pi-sigma @Bartvaderkin see thursday if you can help Jiro out with getting his new mail templates workable for case notifications.

I've showed him the _base.html from mail-editor (https://github.com/maykinmedia/mail-editor/tree/master/mail_editor/templates/mail ) explained add_missing_templates and we tried out zgw_dev_status however this requires a few more steps before Jiro has a way to try out and improve his mail templates

[alextreme]

FAIL: test_send_case_update_email (open_inwoner.openzaak.tests.test_notification_utils.NotificationHandlerUtilsTestCase.test_send_case_update_email)

Traceback (most recent call last):
File "/home/runner/work/open-inwoner/open-inwoner/src/open_inwoner/openzaak/tests/test_notification_utils.py", line 57, in test_send_case_update_email
self.assertIn(case_url, body_html)
AssertionError: '/cases/aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa/status/' not found in

[alextreme]

Can be reviewed after Bart's rebase tests pass

[Bartvaderkin]

I don't know what is going on with the webkit e2e test, I see it failing elsewhere too.

[alextreme]

Feels premature, but merging this in because of the release