Fix #4462: "You have already favorited this post" on upload.

Make adding the fav:self metatag ignore all errors, including when the
post was already favorited before editing the post.

app/models/post.rb

@@ -625,10 +625,10 @@ class Post < ApplicationRecord
           add_pool!(pool) if pool
 
         when /^fav:(.+)$/i
-          add_favorite!(CurrentUser.user)
+          add_favorite(CurrentUser.user)
 
         when /^-fav:(.+)$/i
-          remove_favorite!(CurrentUser.user)
+          remove_favorite(CurrentUser.user)
 
         when /^(up|down)vote:(.+)$/i
           vote!($1)
@@ -792,6 +792,13 @@ class Post < ApplicationRecord
     rescue PostVote::Error
     end
 
+    def remove_favorite(user)
+      remove_favorite!(user)
+      true
+    rescue Favorite::Error
+      false
+    end
+
     # users who favorited this post, ordered by users who favorited it first
     def favorited_users
       favorited_user_ids = fav_string.scan(/\d+/).map(&:to_i)