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/logical/user_name_validator.rb

@@ -7,14 +7,33 @@
 #
 # @see https://guides.rubyonrails.org/active_record_validations.html#custom-validators
 class UserNameValidator < ActiveModel::EachValidator
-  def validate_each(rec, attr, value)
-    name = value
+  ALLOWED_PUNCTUATION = "_.-" # All other punctuation characters are forbidden
 
-    rec.errors.add(attr, "already exists") if User.find_by_name(name).present?
-    rec.errors.add(attr, "must be more than 1 character long") if name.length <= 1
-    rec.errors.add(attr, "must be less than 25 characters long") if name.length >= 25
-    rec.errors.add(attr, "cannot have whitespace or colons") if name =~ /[[:space:]]|:/
-    rec.errors.add(attr, "cannot begin or end with an underscore") if name =~ /\A_|_\z/
-    rec.errors.add(attr, "is not allowed") if name =~ Regexp.union(Danbooru.config.user_name_blacklist)
+  def validate_each(rec, attr, name)
+    forbidden_characters = name.delete(ALLOWED_PUNCTUATION).chars.grep(/[[:punct:]]/).uniq
+
+    if rec.new_record? && User.find_by_name(name).present?
+      rec.errors.add(attr, "already exists")
+    elsif name.length <= 1
+      rec.errors.add(attr, "must be more than 1 character long")
+    elsif name.length >= 25
+      rec.errors.add(attr, "must be less than 25 characters long")
+    elsif name =~ /[[:space:]]/
+      rec.errors.add(attr, "can't contain whitespace")
+    elsif name =~ /\A[[:punct:]]/
+      rec.errors.add(attr, "can't start with '#{name.first}'")
+    elsif name =~ /[[:punct:]]\z/
+      rec.errors.add(attr, "can't end with '#{name.last}'")
+    elsif name =~ /__/
+      rec.errors.add(attr, "can't contain multiple underscores in a row")
+    elsif forbidden_characters.present?
+      rec.errors.add(attr, "can't contain #{forbidden_characters.map { |c| "'#{c}'" }.to_sentence}")
+    elsif name !~ /\A([a-zA-Z0-9]|\p{Han}|\p{Hangul}|\p{Hiragana}|\p{Katakana}|[#{ALLOWED_PUNCTUATION}])+\z/
+      rec.errors.add(attr, "must contain only basic letters or numbers")
+    elsif name =~ /\Auser_\d+\z/i
+      rec.errors.add(attr, "can't be the same as a deleted user")
+    elsif name =~ Regexp.union(Danbooru.config.user_name_blacklist)
+      rec.errors.add(attr, "is not allowed")
+    end
   end
 end