FIX: ensure image tracking custom fields have no dupes

All these columns must never include duplicates for a single post
otherwise code breaks. All are defined in post.rb in the top in constants
but we usually prefer not to ref constants in case they change and migration
becomes inconsistent.

db/migrate/20190424065841_add_post_image_indexes.rb

@@ -0,0 +1,24 @@
+class AddPostImageIndexes < ActiveRecord::Migration[5.2]
+  def change
+
+    %w{
+      large_images
+      broken_images
+      downloaded_images
+    }.each do |field|
+
+      execute <<~SQL
+        DELETE FROM post_custom_fields f
+        WHERE name = '#{field}' AND id > (
+          SELECT MIN(f2.id) FROM post_custom_fields f2
+            WHERE f2.post_id = f.post_id AND f2.name = f.name
+        )
+      SQL
+
+      add_index :post_custom_fields, [:post_id],
+        name: "post_custom_field_#{field}_idx",
+        unique: true,
+        where: "name = '#{field}'"
+    end
+  end
+end