Add project_dir to user_settings properties

[tcompa]

• This is an optional string (representing an absolute path).
• User can always see it, and this exists for all values of FRACTAL_RUNNER_BACKEND. This means that we'll start showing settings for all runner backends.
• user cannot modify this setting
• Admin can edit it (for all runner backends), including unsetting it (i.e. setting it to null)
• Description: "A base folder used for default zarr_dir paths"