Task Notifications don't work with multiple assignees

[abrassat]

Describe the bug
The task notifications should be sent to all assignees.

Steps to reproduce
Steps to reproduce the problematic behavior:

1. Make sure User1 has task notifications enabled
2. Create a task with User1 as assignee
3. Check that User1 receives notifications (try editing the due date and saving, for example)
4. Change the assignee to be both User1 AND User2
5. See that User1 (and User2) doesn't receive notifications about the task

Expected behavior
The task notifications should be sent to all assignees.

Actual behavior
The task notifications aren't sent to anyone.

Environment (please complete the following information):

• OS: linux mint
• Browser firefox
• XWiki version 15.10
• App version 3.10.2
  This bug is likely older, and doesn't have anything to do with the parent upgrade. (I didn't test it on other versions)

Additional context
If the assignee field is XWiki.User1,XWiki.User2 , the notification will be sent to the non-existent user xwiki:XWiki.User1,XWiki.User2, instead of both users. What's missing is a str.split() somewhere in the notification code.