Periodically run the project cleaning script on the Paratext/projects folder

In addition to cleaning Paratext projects during onboarding, it would be good to automatically and periodically clean all the projects in the `Paratext/projects` folder.  When we do this, it may make more sense to run the cleaning script separately, rather than using the onboarding script to invoke the cleaning script.

See also: https://github.com/sillsdev/silnlp/issues/725