* (bug 19944) Link on image thumbnails no longer link to "Media:" namespace in some...

* (bug 19944) Link on image thumbnails no longer link to "Media:" namespace in some cases
* (bug 25670) wfFindFile() now checks the namespace of the given title, only "File" and "Media" are allowed now

The problem with bug 19944 was that the cache in RepoGroup was holding a file with a Title that has its namespace set to NS_MEDIA; the cache is now only used for title with namespace set to NS_FILE.
Also modified ImageGallery to not call wfFindFile() on non-NS_FILE title; was obviously breaking the above fix.