Set config.cache_store in environments file. (#3219)

* Set config.cache_store in application.rb

* Set config.cache_store in environments.

* fix code format.

lib/mastodon/redis_config.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+if ENV['REDIS_URL'].blank?
+  password = ENV.fetch('REDIS_PASSWORD') { '' }
+  host     = ENV.fetch('REDIS_HOST') { 'localhost' }
+  port     = ENV.fetch('REDIS_PORT') { 6379 }
+  db       = ENV.fetch('REDIS_DB') { 0 }
+
+  ENV['REDIS_URL'] = "redis://#{password.blank? ? '' : ":#{password}@"}#{host}:#{port}/#{db}"
+end
+
+namespace = ENV.fetch('REDIS_NAMESPACE') { nil }
+cache_namespace = namespace ? namespace + '_cache' : 'cache'
+REDIS_CACHE_PARAMS = {
+  expires_in: 10.minutes,
+  namespace: cache_namespace,
+}.freeze