Revert "Remove everything related to nonexistent $wgExcludeFromThumbnailPurge"

This reverts commit 951ea09fa03be6f31c2a7eb1fea2f2272ab10df6

Please keep the extension in a consistent state.
Merge "Revert "Remove everything related to nonexistent $wgExcludeFromThumbnailPurge""