upgrade `acts_as_tenant`

[xmunoz]

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• What are we trying to do?
  We can not currently upgrade acts_as_tenant even to the next patch version, because it appears that the maintainer does not follow semver guidelines and has introduced breaking changes in 0.5.2. We are currently on 0.5.1.

• What feature or behavior is this required for?
  Keeping things up to date.

• How could we solve this issue? (Not knowing is okay!)
  Probably need to refactor our use of acts_as_tenant. Changelog: https://github.com/ErwinM/acts_as_tenant/blob/master/CHANGELOG.md#052

• Anything else?
  Nope!