Upgrade search stack

[sergei-maertens]

• [x] Upgrade to django-haystack 3.1.x (supports ES 5.x and 7.x)
• [x] Deploy ES to another VM to reduce load on the app server (docker)
  • [x] Ensure auth is required to access this
  • [x] Set up DNS
  • [x] Update vault variables/config
  • [x] Ensure that vm.max_map_count=262144 is set (see https://docs.ansible.com/ansible/2.9/modules/sysctl_module.html)
• [x] Ensure that ES host/port etc. can be specified through env (already the case with ELASTIC_SEARCH envvar)
• [x] Figure out a way to deal with old website index (maybe disable it?)