Create Redisable#redis (#9633)

* Create Redisable

* Use #redis instead of Redis.current

app/lib/activity_tracker.rb

@@ -4,6 +4,8 @@ class ActivityTracker
   EXPIRE_AFTER = 90.days.seconds
 
   class << self
+    include Redisable
+
     def increment(prefix)
       key = [prefix, current_week].join(':')
 
@@ -20,10 +22,6 @@ class ActivityTracker
 
     private
 
-    def redis
-      Redis.current
-    end
-
     def current_week
       Time.zone.today.cweek
     end