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

config/routes.rb

@@ -254,6 +254,7 @@ Rails.application.routes.draw do
       resources :mutes,      only: [:index]
       resources :favourites, only: [:index]
       resources :reports,    only: [:index, :create]
+      resources :trends,     only: [:index]
 
       namespace :apps do
         get :verify_credentials, to: 'credentials#show'