favorites: fix LockWaitTimeout errors.

Favoriting posts sometimes fails with ActiveRecord::LockWaitTimeout
errors. This happens when a user tries to favorite multiple posts in
short succession. In that case we want the lock to block and wait for
the other favorite to go through, not to immediately fail.

app/models/favorite.rb

@@ -7,7 +7,7 @@ class Favorite < ApplicationRecord
 
   def self.add(post:, user:)
     Favorite.transaction do
-      User.where(:id => user.id).select("id").lock("FOR UPDATE NOWAIT").first
+      User.where(id: user.id).select("id").lock("FOR UPDATE").first
 
       if user.favorite_count >= user.favorite_limit
         raise Error, "You can only keep up to #{user.favorite_limit} favorites. Upgrade your account to save more."
@@ -29,7 +29,7 @@ class Favorite < ApplicationRecord
         post_id = post.id
       end
 
-      User.where(:id => user.id).select("id").lock("FOR UPDATE NOWAIT").first
+      User.where(id: user.id).select("id").lock("FOR UPDATE").first
 
       return unless Favorite.for_user(user.id).where(:user_id => user.id, :post_id => post_id).exists?
       Favorite.for_user(user.id).where(post_id: post_id).delete_all