Add email verification

[Matthias84]

We use email verification for every kind of interaction with public users:

• submit new issues
• public comments
• votes

We need the following functionality

• custom email template incl. inheritance to format IDs, links
• generate secure (private!) hashes
• Link page endpoints to apply model changes and show confirmation
• The verification is invalid after 24hrs

[Matthias84]

Currently we implemented a basic mail notification to the author

• https://stackoverflow.com/questions/55552843/django-send-email-when-object-is-created
• https://django-mail-templated.readthedocs.io/en/master/getting_started.html#creating-templates

Templates need to take header fields into account (e.g. priority)

[Matthias84]

• email templates / hierachy
  • https://stackoverflow.com/a/28476681
  • first txt mails only, first row is subject
• notify issue authors (immedantly), if anything changes
• registration links
  • https://github.com/vinta/django-email-confirm-la
  • https://djangopackages.org/grids/g/invitations/
  • https://github.com/mfogel/django-simple-email-confirmation
  • own confirmation mechanism + cleanup better https://medium.com/@frfahim/django-registration-with-confirmation-email-bb5da011e4ef
• (send batch emails via cron / command)
  • https://medium.com/@harishoraon/writing-your-first-cron-job-in-django-ed62b805d822
  • https://medium.com/@kevin.michael.horan/scheduling-tasks-in-django-with-the-advanced-python-scheduler-663f17e868e6
  • celery not because complex, troublesome and not nessesary for non critical delayed jobs
• mailing 3rd parties / externals
  • tempaling
  • dialog for short-description e.g. ("Beschmierter Briefkasten Ecke Lange Straße / Breite Straße")