adds aria text to indicate course selection is required for conversations

fixes CNVS-19240

note: selection is not mandatory for admins

test plan:
- as a non-admin, go to compose message in coversations
- course menu should read "A course selection is required before entering
  recipients"
- select a course, course menu should now read name of course
- return menu to 'Select course' default, the required statement should return
- required statement should not read out for admins

Change-Id: I1954ef1d48a77990b3dfa111935be3e788f0d0a5
Reviewed-on: https://gerrit.instructure.com/50959
Tested-by: Jenkins
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matt Berns <mberns@instructure.com>

Author: MLBerns

app/coffeescripts/views/conversations/CourseSelectionView.coffee

@@ -52,6 +52,7 @@ define [
       @$el.html(template(data))
       @$el.selectpicker('refresh')
       @$picker.find('.paginatedLoadingIndicator').remove()
+      @getAriaLabel()
       @createSearchViews()
       if !@renderValue() then @loadAll()
 
@@ -85,9 +86,15 @@ define [
       return if @silenced
       @_value = @$el.val()
       @triggerEvent()
+      @getAriaLabel()
       @searchViews.forEach (view) ->
         view.clearSearch()
 
+    getAriaLabel: ->
+      return if _.include(ENV.current_user_roles, 'admin')
+      label = @getCurrentContext().name || I18n.t("Select course: a selection is required before recipients field will become available")
+      @$picker.find('button').attr("aria-label", label)
+
     getCurrentContext: ->
       matches = @_value.match(/(\w+)_(\d+)/)
       return {} unless matches

app/coffeescripts/views/conversations/MessageFormDialog.coffee

@@ -175,7 +175,9 @@ define [
         disabled: @model?.get('private')
       ).render()
       @recipientView.on('changeToken', @recipientIdsChanged)
-      @recipientView.disable(true) unless _.include(ENV.current_user_roles, 'admin')
+      unless _.include(ENV.current_user_roles, 'admin')
+        @$messageCourse.attr('aria-required', true)
+        @recipientView.disable(true)
 
       @$messageCourse.prop('disabled', !!@model)
       @courseView = new CourseSelectionView(

app/views/jst/conversations/MessageFormDialog.handlebars

@@ -32,9 +32,6 @@
         <div class='message-header-label'>
           <label for="compose-message-course">
             {{#t "labels.course"}}Course{{/t}}
-            <span class="screenreader-only">
-              {{#t "labels.course_required"}}You must select a Course before entering recipients.{{/t}}
-            </span>
           </label>
         </div>
         <div class='message-header-input'>