DS-740 Public derivative thumbnails of restricted original bitstreams

https://jira.duraspace.org/browse/DS-740

Adding a config where you specify which MediaFilters to be white-listed to generate publicly accessible derivatives, i.e. thumbnails. In case the original parent bitstream is restricted. Otherwise a visitor might get a broken image thumbnail since its restricted.

```
#Publicly accessible thumbnails of restricted content.
#List the MediaFilter name's that would get publicly accessible permissions
#Any media filters not listed will instead inherit the permissions of the parent bitstream
filter.org.dspace.app.mediafilter.publicPermission = JPEGFilter, XPDF2Thumbnail
```

![image](https://cloud.githubusercontent.com/assets/58014/3109756/366267fa-e69d-11e3-9727-32a7d6367603.png)

By default, this will not be enabled, to preserve existing behavior. But many people that I have talked to were open to having publicly accessible thumbnails of restricted content, nobodies use case is to store Top Secret documents in DSpace, yet.