posts: show length of videos and animations in thumbnails.

Show the length of videos and animated posts in the thumbnail. The
length is shown the top left corner in MM:SS format. This replaces the
play button icon.

Show a speaker icon instead of a music note icon for posts with sound.

Doing this requires doing `.includes(:media_asset)` in a bunch of
places to avoid N+1 queries when we access the post's duration.

app/helpers/application_helper.rb

@@ -102,6 +102,20 @@ module ApplicationHelper
     end
   end
 
+  def duration_to_hhmmss(seconds)
+    hh = seconds.div(1.hour).to_s
+    mm = seconds.div(1.minute).to_s
+    ss = "%.2d" % (seconds.round % 1.minute)
+
+    if seconds >= 1.hour
+      "#{hh}:#{mm}:#{ss}"
+    elsif seconds >= 1.second
+      "#{mm}:#{ss}"
+    else
+      "0:01"
+    end
+  end
+
   def humanized_number(number)
     if number >= 1_000_000
       format("%.1fM", number / 1_000_000.0)