Fix #4574: "unknown keyword: :tag_count" when changing tag category.

Bug: Tag#update_post_category_counts effectively called
`update_all("tag_count_general" => 123, :tag_count => 456)`, which led
to an argument error due to the :tag_count symbol being treated as a
keyword argument.

Fix: Don't mess around with `update_all`, just regenerate the tag counts
then `save!` the post. We have to do this in an after_save callback
because we need the updated category to be in the database for `set_tag_counts`
to work. Delete `fix_post_counts` because it was unused.

This is most likely a regression caused by the upgrade of the
newrelic_rpm gem to 6.12. This can only be reproduced in production when
using Newrelic and it didn't happen before the upgrade. Presumably
Newrelic is splatting the arguments when it calls `update_all`, which
causes symbol keys to be treated as keywords.

app/models/post.rb

@@ -400,24 +400,16 @@ class Post < ApplicationRecord
       set_tag_count(category, self.send("tag_count_#{category}") + 1)
     end
 
-    def set_tag_counts(disable_cache = true)
+    def set_tag_counts
       self.tag_count = 0
       TagCategory.categories.each {|x| set_tag_count(x, 0)}
-      categories = Tag.categories_for(tag_array, :disable_caching => disable_cache)
+      categories = Tag.categories_for(tag_array, disable_caching: true)
       categories.each_value do |category|
         self.tag_count += 1
         inc_tag_count(TagCategory.reverse_mapping[category])
       end
     end
 
-    def fix_post_counts(post)
-      post.set_tag_counts(false)
-      if post.changes_saved?
-        args = Hash[TagCategory.categories.map {|x| ["tag_count_#{x}", post.send("tag_count_#{x}")]}].update(:tag_count => post.tag_count)
-        post.update_columns(args)
-      end
-    end
-
     def merge_old_changes
       reset_tag_array_cache
       @removed_tags = []