Implement the ability for instances to define a list of disallowed hashtags (#7176)

The goal here isn't to prevent these hashtags from existing, but just to strongly curtail their usage; The hashtags may still exist in the database via federated status, or from being created prior to this feature.

app/validators/disallowed_hashtags_validator.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class DisallowedHashtagsValidator < ActiveModel::Validator
+  def validate(status)
+    return unless status.local? && !status.reblog?
+
+    tags = Extractor.extract_hashtags(status.text)
+    tags.keep_if { |tag| disallowed_hashtags.include? tag.downcase }
+
+    status.errors.add(:text, I18n.t('statuses.disallowed_hashtags', tags: tags.join(', '), count: tags.size)) unless tags.empty?
+  end
+
+  private
+
+  def disallowed_hashtags
+    return @disallowed_hashtags if @disallowed_hashtags
+
+    @disallowed_hashtags = Setting.disallowed_hashtags.nil? ? [] : Setting.disallowed_hashtags
+    @disallowed_hashtags = @disallowed_hashtags.split(' ') if @disallowed_hashtags.is_a? String
+    @disallowed_hashtags = @disallowed_hashtags.map(&:downcase)
+  end
+end