Use project slug for generating name of zipfiles, instead of project title

[tompollard]

We generate a zipfile containing all files within a project. At the moment, it looks like the filename of this zipfile is based on the project title. It would make much more sense for the filename to be based on the project slug, I think.

The slug is unique, it doesn't have characters like spaces, apostrophes, etc, and it is short. Titles are much less suited to a filename. The function that needs a rethink is copied below:

https://github.com/MIT-LCP/physionet-build/blob/141d2a9262e6b9191e3bf81f3ccee9485c43e9cf/physionet-django/project/modelcomponents/publishedproject.py#L112-L126