Granular Custom Property access control.

We'd like to have granular control of who can use Custom Property fields based on user class in two different ways:

- Being able to define which Custom Property fields are presented as options to populate when clicking "Build +" to make an installer package.

- Being able to define which user classes are allowed to fill / edit specific Custom Property fields post-install.

There's certain information we'd like to be able to present for the session list that we'd only want editable by Administrators (or, really greater than Tier 1) to prevent our Tier 1 personnel from modifying it.

Notes don't really cut it as a workaround unless we lock those down to only be accessible to administrators.  (Plus, Notes, as currently implemented, would need to let us do stuff like locking / dragging and dropping key notes to the top of the notes list as needed to keep things presented consistently.)