Track trending tags (#7638)

* Track trending tags

- Half-life of 1 day
- Historical usage in daily buckets (last 7 days stored)
- GET /api/v1/trends

Fix #271

* Add trends to web UI

* Don't render compose form on search route, adjust search results header

* Disqualify tag from trends if it's in disallowed hashtags setting

* Count distinct accounts using tag, ignore silenced accounts

app/serializers/rest/tag_serializer.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class REST::TagSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :name, :url, :history
+
+  def url
+    tag_url(object)
+  end
+end