Test database upgrades

[vmcj]

Currently the database upgrade is not tested:

• Install an older (un)supported release via the documentation
• Do some minimal actions:
  • Create a team
  • Create a clarification
  • Answer it
  • Upload a problem
  • Submit a problem
  • Create some extra entities and delete those for example:
    • Create a contest,
    • With a team submit
    • Delete the contest again

Keep the dump small, upgrade tests can be fast as we don't need a lot of problems/submissions but rather more (rarely used) functions/features tested.

When we have this dump we can make an CI job to test the upgrade with ./bin/dj_setup_database upgrade.