Fix Demo mode issues

[mathemancer]

Fixes #3433

This fixes a configuration issue that prevented demo mode from working properly in the previous release.

Technical details

The main problem was that the template setup was using an old, no-longer-working way of addressing the database. The broken template was causing creating of demo databases to fail.

This PR also adds a step to install the demo superuser when running in demo mode.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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