Resources "Overwrite Existing" should default to false for Drop Box uploads

Overwrite existing should not be the default for Drop Box uploads as this will remove existing uploads which is usually not what is desired.

Use Case is the following:

Our request came from curriculum support staff that were uploading back to the students dropbox with instructor’s comments added and they did not want to overwrite the student’s original submission. When uploading back 90 or so students they would forget to uncheck the box and overwrite the original submission which they wanted to preserve. 

If students wanted to overwrite their submission they could check the box.