Add multiple assignees to issue board

What does this MR do?

Adds multiple assignees to issue board. Backend is incomplete in this MR. It is only stubbed for the rendering.

Are there points in the code the reviewer needs to double check?

Shouldn't be.

Why was this MR needed?


Screenshots (if relevant)

1 Assignee 2 Assignees 3 Assignees
Screen_Shot_2017-03-28_at_2.01.14_PM Screen_Shot_2017-03-28_at_2.01.47_PM Screen_Shot_2017-03-28_at_2.02.09_PM
+1 +10 99+
Screen_Shot_2017-03-28_at_2.00.01_PM Screen_Shot_2017-03-28_at_2.09.51_PM Screen_Shot_2017-03-28_at_2.10.32_PM

What are the relevant issue numbers?