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/controllers/api/v1/trends_controller.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class Api::V1::TrendsController < Api::BaseController
+  before_action :set_tags
+
+  respond_to :json
+
+  def index
+    render json: @tags, each_serializer: REST::TagSerializer
+  end
+
+  private
+
+  def set_tags
+    @tags = TrendingTags.get(limit_param(10))
+  end
+end