[Notifications] Don't use the group name as the sender

[DeteesWuber]

Problem statement:

The group name is displayed as the sender of a group message. This can make it look like spam if creative group names are chosen.

Proposed solution:

The sender of a notification should be the person who posted the message or no person at all, just ‘notifications@instructure.com’ (it would be nice if somewhere in the body text of the message there was a reference to which group the message came from and in which course this group is based).

User role(s):

admin,instructor,student