a11y workaround assignment "assign to"
a11y workaround assignment "assign to"

The assignment "Assign to" field is implemented in a react app that
utilizes react-tokeninput. When focusing on the field, focus jumps to
remove button of last entered asignee, which prevents non-visual users
from knowing what the field is for.

I attempted a few different approaches to solving this problem using the
react-tokeninput component as it exists now, but no dice. We should add
functionality to the component to hint context on focus, but for now, I
added tabindex on the label field above the field to cue context.

Test Plan:
  * Open assignment edit page
  * Verify that tab stops on "assign to" field, and various screen
    readers read text properly

closes CNVS-23041

Change-Id: I76d879eceee46f691c712293ba44e089abe31e32
Reviewed-on: https://gerrit.instructure.com/63952
Reviewed-by: Matthew Wheeler <ce71fbafda91d6c8f12582e1837687d5fe592dd9@instructure.com>
Tested-by: Jenkins
QA-Review: Adam Stone <a0b2b61e2edcf99aa16080c2254a6ae6923d1fac@instructure.com>
Product-Review: Aaron Cannon <25919c673621c006b735dcd5a28df63325948e2b@instructure.com>

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