Finish extra files

[acbart]

Students and instructors can both have extra files along with their submission. Instructors in particular can have a wide set of possible files:

• Instructor (!): Invisible to the student under all circumstances
• Start Space (^): Used to reset the student namespace
• Student Space (): Visible to the student when display.hideFiles is not true, able to be edited
• Hidden Space (?): Not directly visible to the student, but accessible programmatically
• Read-only Space (&): An instructor file type visible to the student, but is uneditable by them
• Secret Space ($): Not visible from the menu at all, some other mechanism controls it
• Generated Space (*): Visible to the student, but destroyed after Engine.Clear. Can shadow an actual file.
• Concatenated Space (#): Used when bundling a space for the server.