Add trending links (#16917)

* Add trending links

* Add overriding specific links trendability

* Add link type to preview cards and only trend articles

Change trends review notifications from being sent every 5 minutes to being sent every 2 hours

Change threshold from 5 unique accounts to 15 unique accounts

* Fix tests

app/models/trends/history.rb

@@ -0,0 +1,98 @@
+# frozen_string_literal: true
+
+class Trends::History
+  include Enumerable
+
+  class Aggregate
+    include Redisable
+
+    def initialize(prefix, id, date_range)
+      @days = date_range.map { |date| Day.new(prefix, id, date.to_time(:utc)) }
+    end
+
+    def uses
+      redis.mget(*@days.map { |day| day.key_for(:uses) }).map(&:to_i).sum
+    end
+
+    def accounts
+      redis.pfcount(*@days.map { |day| day.key_for(:accounts) })
+    end
+  end
+
+  class Day
+    include Redisable
+
+    EXPIRE_AFTER = 14.days.seconds
+
+    def initialize(prefix, id, day)
+      @prefix = prefix
+      @id     = id
+      @day    = day.beginning_of_day
+    end
+
+    attr_reader :day
+
+    def accounts
+      redis.pfcount(key_for(:accounts))
+    end
+
+    def uses
+      redis.get(key_for(:uses))&.to_i || 0
+    end
+
+    def add(account_id)
+      redis.pipelined do
+        redis.incrby(key_for(:uses), 1)
+        redis.pfadd(key_for(:accounts), account_id)
+        redis.expire(key_for(:uses), EXPIRE_AFTER)
+        redis.expire(key_for(:accounts), EXPIRE_AFTER)
+      end
+    end
+
+    def as_json
+      { day: day.to_i.to_s, accounts: accounts.to_s, uses: uses.to_s }
+    end
+
+    def key_for(suffix)
+      case suffix
+      when :accounts
+        "#{key_prefix}:#{suffix}"
+      when :uses
+        key_prefix
+      end
+    end
+
+    def key_prefix
+      "activity:#{@prefix}:#{@id}:#{day.to_i}"
+    end
+  end
+
+  def initialize(prefix, id)
+    @prefix = prefix
+    @id     = id
+  end
+
+  def get(date)
+    Day.new(@prefix, @id, date)
+  end
+
+  def add(account_id, at_time = Time.now.utc)
+    Day.new(@prefix, @id, at_time).add(account_id)
+  end
+
+  def aggregate(date_range)
+    Aggregate.new(@prefix, @id, date_range)
+  end
+
+  def each(&block)
+    if block_given?
+      (0...7).map { |i| block.call(get(i.days.ago)) }
+    else
+      to_enum(:each)
+    end
+  end
+
+  def as_json(*)
+    map(&:as_json)
+  end
+end