[Gradebook] Single Email & BCC default for "Message students who"

[Draco]

Problem statement:

Currently, when using "Message students who" from the gradebook, there is a possibility of multiple students being messaged. Those messages show up as single emails in the "Send" folder. If we have multiple students and/or multiple assignments and/or multiple classes to send messages to, this floods our "send" folder.

Proposed solution:

Keep these messages aggregated using BCC as default for each instance. Then, there is only one message in our send box, for each instance, and it's easier to keep track of students with missing assignments.

User role(s):

instructor,designer