Previews of restricted files.

[kosarko]

We generally should not show (or maybe even generate) previews of restricted files.

Originally posted by @kosarko in #1316 (comment)

Why is the authorization false in

clarin-dspace/dspace-server-webapp/src/main/java/org/dspace/app/rest/repository/MetadataBitstreamRestRepository.java

Line 110 in 04d64f7

boolean canPreview = previewContentService.canPreview(context, bitstream, false);

and

clarin-dspace/dspace-api/src/main/java/org/dspace/scripts/filepreview/FilePreview.java

Line 149 in 7c920b2

boolean canPreview = previewContentService.canPreview(context, bitstream, false);

?

[kosarko]

ping #1271

[kuchtiak-ufal]

Pull Request: #1327