users: remove 'spoilers' tag from default blacklist.

Rationale:

* The spoilers tag is the most frequently removed tag from the default blacklist.
* It's frustrating for regular users to have posts randomly hidden because of trivial
  spoilers from a series they don't care about.
* The spoilers tag is used way too liberally for things that aren't considered
  spoilers on other sites.
* If you're looking up fanart on the internet, you should expect to see a certain
  level of spoilers.
* The tag is used very inconsistently, with some characters like Nia_(blade)_(xenoblade)
  getting the spoilers tag half the time and the rest of the time not.

app/models/user.rb

@@ -67,7 +67,7 @@ class User < ApplicationRecord
     is_verified
   ]
 
-  DEFAULT_BLACKLIST = ["spoilers", "guro", "scat", "furry -rating:s"].join("\n")
+  DEFAULT_BLACKLIST = ["guro", "scat", "furry -rating:s"].join("\n")
 
   attribute :id
   attribute :created_at