users: add stricter username rules.

Add stricter username rules:

* Only allow usernames to contain basic letters, numbers, CJK characters, underscores, dashes and periods.
* Don't allow names to start or end with punctuation.
* Don't allow names to have multiple underscores in a row.
* Don't allow active users to have names that look like deleted users (e.g. "user_1234").
* Don't allow emoji or any other Unicode characters except for Chinese, Japanese, and Korean
  characters. CJK characters are currently grandfathered in but will be disallowed in the future.

Users with an invalid name will be shown a permanent sitewide banner until they change their name.

app/models/user.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class User < ApplicationRecord
+  extend Memoist
+
   class PrivilegeError < StandardError; end
 
   module Levels
@@ -204,6 +206,18 @@ class User < ApplicationRecord
         errors.add(:base, "Can't enable privacy mode without a Gold account")
       end
     end
+
+    def name_errors
+      User.validators_on(:name).each do |validator|
+        validator.validate_each(self, :name, name)
+      end
+
+      errors
+    end
+
+    def name_invalid?
+      name_errors.present?
+    end
   end
 
   concerning :AuthenticationMethods do
@@ -695,4 +709,6 @@ class User < ApplicationRecord
   def self.available_includes
     [:inviter]
   end
+
+  memoize :name_errors
 end