A11y: Differentiated reply buttons for SR users in discussions

Fixes CNVS-14714

Test plan:
- Visit a discussion with replies (both threaded and not).
- With a screen reader, verify that the reply buttons specify whether
  they reply to the main discussion post, or to a comment.

Change-Id: If262dcc5726bf7cf9ea9ff4cf4b599da60def399
Reviewed-on: https://gerrit.instructure.com/48006
Tested-by: Jenkins
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>

app/coffeescripts/views/DiscussionTopic/TopicView.coffee

@@ -166,6 +166,7 @@ define [
         modelData = @model.toJSON()
         modelData.showBoxReplyLink = true
         modelData.root = true
+        modelData.isForMainDiscussion = true
         html = replyTemplate modelData
         @$('#discussion_topic').append html
       super

app/views/jst/discussions/_entry_content.handlebars

@@ -74,7 +74,8 @@
      data-event="addReply"
      href="#">
     <i class="icon-replied"></i>
-    {{#t "write_a_reply"}}Reply{{/t}}
+    <span aria-hidden="true">{{#t "write_a_reply"}}Reply{{/t}}</span>
+    <span class="screenreader-only">{{#t}}Reply to Comment{{/t}}</span>
   </a>
 </div>
 {{/if}}

app/views/jst/discussions/_reply_form.handlebars

@@ -3,7 +3,12 @@
     <div class="notification" data-bind="notification"></div>
     <a class="discussion-reply-action discussion-reply-box" data-event="addReply" href="#">
       <i class="icon-replied"></i>
-      {{#t "write_a_reply"}}Reply{{/t}}
+      <span aria-hidden="true">{{#t "write_a_reply"}}Reply{{/t}}</span>
+      {{#if isForMainDiscussion}}
+        <span class="screenreader-only">{{#t}}Reply to Main Discussion{{/t}}</span>
+      {{else}}
+        <span class="screenreader-only">{{#t}}Reply to Comment{{/t}}</span>
+      {{/if}}
     </a>
   </div>
 {{/if}}