Rework schleuder_web setup to make it more deterministic

[nevart]

The current state of things are quite brittle, specifically how the upstream git tag is checked out and the mechanism how to check for potential updates. This needs to be improved.

[nevart]

Existing installations might run into a problem, if handling schleuder-web, due to "dubious ownership" of the checked out repo.

[nevart]

Further, it seems, that the task to checkout a specific version of the schleuder-web repo doesn't work as expected. If a checkout already exists, it might stay on the old, previous, wrong version, instead of the new version as defined via the relevant variable.