Lock project option available to authors to ensure they don't accidentally edit finished projects

[FayCross]

An idea that came up in one of the Xerte conference sessions:

The ability to lock a project so it becomes read-only even to project authors/editors until you turn this off. Would be useful to guarantee that you aren't going to accidentally change a finished project and also for people who are getting students to submit projects as coursework it would ensure that students aren't changing the project after the submission date (the locked date would need to be shown in the project properties)