Add gif auto-play/pause preference

This introduces a new per-user preference called
"Auto-play animated GIFs", which is enabled by default. When a
user disables this setting, gifs in toots become click-to-play.

Previews of animated gifs were changed to display the video play
button so that users can distinguish them from regular images.

This setting also affects account avatars in the detailed account
view, which was changed to use the same hover-to-play mechanism
that is used for animated avatars in timelines.

Fixes #1652

app/models/user.rb

@@ -32,4 +32,8 @@ class User < ApplicationRecord
   def setting_boost_modal
     settings.boost_modal
   end
+
+  def setting_auto_play_gif
+    settings.auto_play_gif
+  end
 end