Add aria-label to articles page's CTA (forem#1004)

In the main articles page CTA for signing in via Twitter or Github, previously, screen readers say "Twitter" and "GitHub."  I added an `aria-label` attribute so that screen readers will say "Sign in with Twitter" and "Sign in with GitHub."  Tested on Windows 7, in Chrome, using Chrome Vox extension, and works well!

app/views/articles/_main_stories_feed.html.erb

@@ -12,10 +12,10 @@
               <% end %>
             </h2>
             <div class="button-container">
-              <a href="/users/auth/twitter?callback_url=<%= ApplicationConfig["APP_PROTOCOL"] %><%= ApplicationConfig["APP_DOMAIN"] %>/users/auth/twitter/callback" class="cta cta-button" data-no-instant>
+              <a href="/users/auth/twitter?callback_url=<%= ApplicationConfig["APP_PROTOCOL"] %><%= ApplicationConfig["APP_DOMAIN"] %>/users/auth/twitter/callback" class="cta cta-button" aria-label="Sign in with Twitter." data-no-instant>
                 TWITTER
               </a>
-              <a href="/users/auth/github?state=in-feed-cta" class="cta cta-button" data-no-instant>
+              <a href="/users/auth/github?state=in-feed-cta" class="cta cta-button" aria-label="Sign in with GitHub." data-no-instant>
                 GITHUB
               </a>
             </div>