subject mail populated when user shares >>file/folder<<

in response to @rperezb issue #767 , the subject mail is now populated when the user shares either a file or folder. I also kept the same syntax as it is on the web, with file/folder name surrounded with greater and less signs. 
Concatenating strings is problematic for translations to other langauges, Please, define a single string resource for the subject that includes format markers, and then replace them here using ```String.format(...)```