Replace Jupyter file-shares with generic project-level file-shares

[MoritzWeber0]

One target of the Collaboration Manager is to replace the Jupyter specific parts of the backend with a generic implementation.

Right now, Jupyter file-shares have an additional layer for tool models (Users have to create a tool models with tool Jupyter in the project). This is not very intuitive. Instead, it should be possible to have one project-level file-share (optional).

As a consequence, the project-level file-shares would be mounted in all persistent session containers, not just the Jupyter containers. Also, the mount should not include the additional layer anymore: shared / project instead of shared / project / model.

The file-share should be visible below the models overview in projects the projects detail page and should display the size of the file-share. If a file-share doesn't exist, project leads and administrators should be able to create it.