Separate core and default distribution

We can now separate the AlekSIS core from the default AlekSIS distribution as we now have a private package registry to iinstall dependencies from.

• Remove submodules of apps and the apps/ directory
• Rename this project as well as the PyPI package to AlekSIS-Core
• Change dependencies on AlekSIS to AlekSIS-Core in all apps
• Make poetry depend on packages from EduGit registry only when building in CI
• Create new project AlekSIS for the default distribution
• Move all parts of the default distribution to the new project (including Git history, if possible)
• Move all issues regarding distribution
• Ensure core builds, all apps build, and distribution builds
• Ensure app tests are self-contained
• Ensure pipeline builds are run in the correct order across projects
• Update project and source code locations on OpenHub