posts: choose better images for opengraph previews.

When choosing the Open Graph image (the preview image shown when a
Danbooru link is posted on Discord or social media), choose the safest
image with the highest score, rather than the image with the highest
favcount.

app/models/post.rb

@@ -305,6 +305,11 @@ class Post < ApplicationRecord
       flags.join(" ")
     end
 
+    # g => 0, s => 1, q => 2, e => 3
+    def rating_id
+      RATINGS.keys.index(rating)
+    end
+
     def pretty_rating
       RATINGS.fetch(rating)
     end