Contributions: default material protection

[jbenito]

[Requested by Martine Desnyder]

Some event organizers would like that all files that are uploaded to any contribution have a specific protection. The use case would be:

1. Event manager does setup the event in order to force a given protection (e.g. e-group foo-egroup) for all uploaded files to any contribution.
2. Submitter uploads a file (any material type) and the material is automatically protected following the setup in the previous step 1.

Now the question is, should the submitter be able to change the protection afterwards?