Allow plugins to customize the columns returned by user_actions

This is useful if they have a different mechanism for rendering an
avatar and want to include custom columns via the `custom_avatar_column`
plugin function.

app/models/user_action.rb

@@ -167,6 +167,18 @@ SQL
     offset = opts[:offset] || 0
     limit = opts[:limit] || 60
 
+    # Acting user columns. Can be extended by plugins to include custom avatar
+    # columns
+    acting_cols = [
+      'u.id AS acting_user_id',
+      'u.name AS acting_name'
+    ]
+
+    AvatarLookup.lookup_columns.each do |c|
+      next if c == :id || c['.']
+      acting_cols << "u.#{c} AS acting_#{c}"
+    end
+
     # The weird thing is that target_post_id can be null, so it makes everything
     #  ever so more complex. Should we allow this, not sure.
     builder = SqlBuilder.new <<-SQL
@@ -179,8 +191,7 @@ SQL
         p.reply_to_post_number,
         pu.username, pu.name, pu.id user_id,
         pu.uploaded_avatar_id,
-        u.username acting_username, u.name acting_name, u.id acting_user_id,
-        u.uploaded_avatar_id acting_uploaded_avatar_id,
+        #{acting_cols.join(', ')},
         coalesce(p.cooked, p2.cooked) cooked,
         CASE WHEN coalesce(p.deleted_at, p2.deleted_at, t.deleted_at) IS NULL THEN false ELSE true END deleted,
         p.hidden,