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/services/process_hashtags_service.rb

@@ -4,8 +4,10 @@ class ProcessHashtagsService < BaseService
   def call(status, tags = [])
     tags = Extractor.extract_hashtags(status.text) if status.local?
 
-    tags.map { |str| str.mb_chars.downcase }.uniq(&:to_s).each do |tag|
-      status.tags << Tag.where(name: tag).first_or_initialize(name: tag)
+    tags.map { |str| str.mb_chars.downcase }.uniq(&:to_s).each do |name|
+      tag = Tag.where(name: name).first_or_create(name: name)
+      status.tags << tag
+      TrendingTags.record_use!(tag, status.account, status.created_at)
     end
   end
 end