don't load all contexts for section recipient lookup

fixes CNVS-15797

test plan
tl;dr - regression test recipient finder, it should be faster for
 sections

- as a teacher with many courses in many accounts (10ish? as long
 as it usually causes a delay in the address book), begin composing
 a message in the conversations inbox
- select a course from the context dropdown
- using the recipient finder, select 'Course Sections'
- select a section
- ensure that the delay between selecting the section and seeing
 the section users is less than before this commit
- ensure that the section user list has the same contents as
 before this commit

Change-Id: I5ede3d05c25580d9cb354dfd5a4bd03ed6a2e440
Reviewed-on: https://gerrit.instructure.com/41755
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>