A11Y: Include username in aria-label of post region

I think the author of a post is a critical piece of information that
should be communicated right when the post region is reached.

app/assets/javascripts/discourse/app/widgets/post.js

@@ -652,6 +652,7 @@ createWidget("post-article", {
     return {
       "aria-label": I18n.t("share.post", {
         postNumber: attrs.post_number,
+        username: attrs.username,
       }),
       role: "region",
       "data-post-id": attrs.id,

config/locales/client.en.yml

@@ -136,7 +136,7 @@ en:
       to_placeholder: "to date"
     share:
       topic_html: 'Topic: <span class="topic-title">%{topicTitle}</span>'
-      post: "post #%{postNumber}"
+      post: "post #%{postNumber} by @%{username}"
       close: "close"
       twitter: "Share on Twitter"
       facebook: "Share on Facebook"