uses generic user avatar for old conversations "add people" dialog

fixes CNVS-11508

test plan
- open "Add People" dialog on a conversation from the old
 conversations interface
- search for a student that doesn't have an avatar set
- ensure that the generic person avatar is shown

Change-Id: I9d23b673b4963f490373440e9532993f8b337819
Reviewed-on: https://gerrit.instructure.com/31002
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>