[Inbox] Allow observers to message all teachers at once in one Canvas message

[resartoris]

Problem statement:

Allow observers to message all teachers at once in one Canvas message. If a parent needs to send a message, it makes them choose a course and only allows them to send the message to one teacher at a time. 

Proposed solution:

All teachers and observers need a way to send messages to people in different courses, if needed.

User role(s):

observers