Add counter caches for a large performance increase on API requests

app/controllers/api/v1/accounts_controller.rb

app/controllers/api/v1/blocks_controller.rb

app/controllers/api/v1/favourites_controller.rb

app/controllers/api/v1/follow_requests_controller.rb

app/controllers/api/v1/mutes_controller.rb

app/controllers/api/v1/notifications_controller.rb

app/controllers/api/v1/statuses_controller.rb

app/models/favourite.rb

app/models/follow.rb

app/models/status.rb

app/views/api/v1/statuses/_show.rabl

db/migrate/20170330021336_add_counter_caches.rb

db/schema.rb