[Weblate] Update Django 1.11.x -> 2.2.y

Originally created by @hefee on #17335 (Redmine)

We use currently use Django 1.11.26 but we could switch to 2.2.y, as both versions are supported by Weblate.

Django has currently two LTS versions

• 1.11 is supported until April 2020
• 2.2 is supported until April 2022 (because if CVE-2019-19118, we should install 2.2.y with y > 7)

Weblate 4.0 won’t support 1.11 anymore, what is scheduled for April next year see weblate#2201.

We need to check what new depdendencies of Django 2.2 needs to be added to puppet.

Steps for the upgrade:

• Disable runs of cron.sh.
• Remove access to Weblate web interface.
  • Schedule downtime in Icinga Web.
  • Disable Puppet agent.
  • Stop Apache web server.
• Backup repositories and database.
• Upgrade Django to 2.2 (run python -Wa manage.py test before/after each upgrade):
• Check deployment: manage.py check --deploy
• Enable access to Weblate web interface.
  • Start Apache web server.
  • Enable Puppet agent.
• Enable runs of cron.sh.