Change admin UI for hashtags and add back whitelisted trends (#11490)

Fix #271

Add back the `GET /api/v1/trends` API with the caveat that it does
not return tags that have not been allowed to trend by the staff.

When a hashtag begins to trend (internally) and that hashtag has
not been previously reviewed by the staff, the staff is notified.

The new admin UI for hashtags allows filtering hashtags by where
they are used (e.g. in the profile directory), whether they have
been reviewed or are pending reviewal, they show by how many people
the hashtag is used in the directory, how many people used it
today, how many statuses with it have been created today, and it
allows fixing the name of the hashtag to make it more readable.

The disallowed hashtags feature has been reworked. It is now
controlled from the admin UI for hashtags instead of from
the file `config/settings.yml`

app/models/tag.rb

@@ -3,11 +3,16 @@
 #
 # Table name: tags
 #
-#  id         :bigint(8)        not null, primary key
-#  name       :string           default(""), not null
-#  created_at :datetime         not null
-#  updated_at :datetime         not null
-#  score      :integer
+#  id                  :bigint(8)        not null, primary key
+#  name                :string           default(""), not null
+#  created_at          :datetime         not null
+#  updated_at          :datetime         not null
+#  score               :integer
+#  usable              :boolean
+#  trendable           :boolean
+#  listable            :boolean
+#  reviewed_at         :datetime
+#  requested_review_at :datetime
 #
 
 class Tag < ApplicationRecord
@@ -22,16 +27,17 @@ class Tag < ApplicationRecord
   HASHTAG_RE = /(?:^|[^\/\)\w])#(#{HASHTAG_NAME_RE})/i
 
   validates :name, presence: true, format: { with: /\A(#{HASHTAG_NAME_RE})\z/i }
+  validate :validate_name_change, if: -> { !new_record? && name_changed? }
 
-  scope :discoverable, -> { joins(:account_tag_stat).where(AccountTagStat.arel_table[:accounts_count].gt(0)).where(account_tag_stats: { hidden: false }).order(Arel.sql('account_tag_stats.accounts_count desc')) }
-  scope :hidden, -> { where(account_tag_stats: { hidden: true }) }
+  scope :reviewed, -> { where.not(reviewed_at: nil) }
+  scope :pending_review, -> { where(reviewed_at: nil).where.not(requested_review_at: nil) }
+  scope :discoverable, -> { where.not(listable: false).joins(:account_tag_stat).where(AccountTagStat.arel_table[:accounts_count].gt(0)).order(Arel.sql('account_tag_stats.accounts_count desc')) }
   scope :most_used, ->(account) { joins(:statuses).where(statuses: { account: account }).group(:id).order(Arel.sql('count(*) desc')) }
 
   delegate :accounts_count,
            :accounts_count=,
            :increment_count!,
            :decrement_count!,
-           :hidden?,
            to: :account_tag_stat
 
   after_save :save_account_tag_stat
@@ -48,6 +54,40 @@ class Tag < ApplicationRecord
     name
   end
 
+  def usable
+    boolean_with_default('usable', true)
+  end
+
+  alias usable? usable
+
+  def listable
+    boolean_with_default('listable', true)
+  end
+
+  alias listable? listable
+
+  def trendable
+    boolean_with_default('trendable', false)
+  end
+
+  alias trendable? trendable
+
+  def requires_review?
+    reviewed_at.nil?
+  end
+
+  def reviewed?
+    reviewed_at.present?
+  end
+
+  def requested_review?
+    requested_review_at.present?
+  end
+
+  def trending?
+    TrendingTags.trending?(self)
+  end
+
   def history
     days = []
 
@@ -117,4 +157,8 @@ class Tag < ApplicationRecord
     return unless account_tag_stat&.changed?
     account_tag_stat.save
   end
+
+  def validate_name_change
+    errors.add(:name, I18n.t('tags.does_not_match_previous_name')) unless name_was.mb_chars.casecmp(name.mb_chars).zero?
+  end
 end