posts: fix double deletion bug.

Fix a bug where, if a user a was deleting a post and they accidentally
clicked the delete button twice, it could create two flags.

test/factories/post_flag.rb

@@ -1,7 +1,7 @@
 FactoryBot.define do
   factory(:post_flag) do
     creator
-    post { build(:post, is_flagged: true) }
+    post { build(:post) }
     reason {"xxx"}
   end
 end