posts: stop updating fav_string attribute.

Stop updating the fav_string attribute on posts. The column still exists
on the table, but is no longer used or updated.

Like the pool_string in 7d503f08, the fav_string was used in the past to
facilitate `fav:X` searches. Posts had a hidden fav_string column that
contained a list of every user who favorited the post. These were
treated like fake hidden tags on the post so that a search for `fav:X`
was treated like a tag search.

The fav_string attribute has been unused for search purposes for a while
now. It was only kept because of technicalities that required
departitioning the favorites table first (340e1008e) before it could be
removed. Basically, removing favorites with `@favorite.destroy` was
slow because Rails always deletes object by ID, but we didn't have an
index on favorites.id, and we couldn't easily add one until the
favorites table was departitioned.

Fixes #4652. See https://github.com/danbooru/danbooru/issues/4652#issuecomment-754993802
for more discussion of issues caused by the fav_string (in short: write
amplification, post table bloat, and favorite inconsistency problems).

app/controllers/favorites_controller.rb

@@ -1,6 +1,5 @@
 class FavoritesController < ApplicationController
   respond_to :html, :xml, :json, :js
-  rescue_with Favorite::Error, status: 422
 
   def index
     authorize Favorite
@@ -18,23 +17,18 @@ class FavoritesController < ApplicationController
   end
 
   def create
-    authorize Favorite
-    @post = Post.find(params[:post_id])
-    @post.add_favorite!(CurrentUser.user)
-    flash.now[:notice] = "You have favorited this post"
+    @favorite = authorize Favorite.new(post_id: params[:post_id], user: CurrentUser.user)
+    @favorite.save
+    @post = @favorite.post.reload
 
+    flash.now[:notice] = "You have favorited this post"
     respond_with(@post)
   end
 
   def destroy
-    authorize Favorite
-    @post = Post.find_by_id(params[:id])
-
-    if @post
-      @post.remove_favorite!(CurrentUser.user)
-    else
-      Favorite.remove(post_id: params[:id], user: CurrentUser.user)
-    end
+    @favorite = authorize Favorite.find_by!(post_id: params[:id], user: CurrentUser.user)
+    @favorite.destroy
+    @post = @favorite.post.reload
 
     flash.now[:notice] = "You have unfavorited this post"
     respond_with(@post)