Add Notifications for Drop Box activity

See the umbrella Jira for this issue: https://sakaiproject.atlassian.net/browse/SAK-50034

The Drop Box tool should generate notifications for new activity.

Drop Box is similar to Resources, and I know that Resources Notifications were turned off intentionally, since the Resources tool tended to generate too many notifications that were not really useful for end users. However, the purpose of the Drop Box tool is different than Resources. It is intended as a file-sharing space for students and faculty. With that use case in mind, I think that notifications for Drop Box activity might be useful.

As a student, I think it would be useful to get a notification telling me when my instructor has uploaded a file to my Drop Box.

Likewise, as an instructor, if you are using Drop Box as an alternative to Assignments for collecting files from students, it would be useful to know when you have new student files awaiting your review.

Additionally, if users did not want to receive Drop Box notifications, they could always turn off the notifications for that tool from Preferences. (See associated Jira for proposed Notification Preferences modifications.)