Setup share notification

When sharing a file or a folder, user can enable share notifications.

When edit the share, can modify the notification.

There are 3 notifications. Assume user1 is the owner of the share. And the folder is shared to user2. Here are how the notification works:

• Send email notification when file is downloaded: when user2 downloads files from the share, send email notification to user1. 
• Send email notification when file is uploaded: when user2 uploads files to the share, send email notification to user1.
• Notify user when file/folder is changed: when user1 updates files on the shared folder, notify user2 about the change