add activity if public shared file/folder was downloaded

Add activity notification if a public shared file/folder was downloaded

fix #12928 
fix https://github.com/owncloud/activity/issues/103

cc @nickvergessen 
OK, now I see your point. I just discovered something different. I changed it to use the basename for scrollto. The link gets constructed to ```http://localhost/oc/index.php/apps/files?dir=%2FNew+folder&scrollto=welcome.txt``` what is correct in my example.

But he activity app changes it to ```http://localhost/oc/index.php/apps/files?dir=%2F&scrollto=welcome.txt```