fix focus for Add Groups dialog.

fixes CNVS-24876

test plan:
-as a teacher, to to an assignments page
-attempt to add group by clicking +Group button
-Focus should be set on the input field
-input field should properly read out its label
-Make sure it works properly across all screen readers

Change-Id: I199c01ce97d21e5e511355aac35ab2b0dc323246
Reviewed-on: https://gerrit.instructure.com/68671
Tested-by: Jenkins
Reviewed-by: John Corrigan <jcorrigan@instructure.com>
QA-Review: Rajkumar Kethavath <rkethavath@instructure.com>
Product-Review: Jason Sparks <jsparks@instructure.com>

app/coffeescripts/views/assignments/CreateGroupView.coffee

@@ -144,5 +144,6 @@ define [
         @setDimensions(this.defaults.width, SHORT_HEIGHT)
 
       super
+      @$el.find("input:first").focus()
       @checkGroupWeight()
       @getNeverDrops()