*Enhancement* Download files as Read-only on sync

[jmichel-sbctc]

I did some testing regarding the issue #143 and it seems if we have the files download as Read-only when synced it will force the students to do a save-as instead of a normal save when working on files directly from the LMS application. This will then prevent student work from being written over when running the sync application.

[frankyrumple]

Code added to clear/set the read-only flag when downloading files from canvas. When opening files from cache, it forces a save as now.