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/models/status.rb

@@ -59,6 +59,7 @@ class Status < ApplicationRecord
   validates :uri, uniqueness: true, presence: true, unless: :local?
   validates :text, presence: true, unless: -> { with_media? || reblog? }
   validates_with StatusLengthValidator
+  validates_with DisallowedHashtagsValidator
   validates :reblog, uniqueness: { scope: :account }, if: :reblog?
 
   default_scope { recent }

app/validators/disallowed_hashtags_validator.rb

+# 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

config/locales/en.yml

@@ -684,6 +684,9 @@ en:
         one: "%{count} video"
         other: "%{count} videos"
     content_warning: 'Content warning: %{warning}'
+    disallowed_hashtags:
+      one: 'contained a disallowed hashtag: %{tags}'
+      other: 'contained the disallowed hashtags: %{tags}'
     open_in_web: Open in web
     over_character_limit: character limit of %{max} exceeded
     pin_errors:

config/settings.yml

@@ -47,6 +47,7 @@ defaults: &defaults
     - root
     - webmaster
     - administrator
+  disallowed_hashtags: # space separated string or list of hashtags without the hash
   bootstrap_timeline_accounts: ''
   activity_api_enabled: true
   peers_api_enabled: true