Ensure rake tasks that don't need a DB connection can be run without one

When using ActiveRecord::Base.connection.active? without a DB
connection, we get a "PG::ConnectionBad: could not connect to server"
error because ActiveRecord::Base.connection doesn't exist. By using
ActiveRecord::Base.connected? we ensure we don't get this error if
the connection is missing, which is the all point of the
Gitlab::CurrentSettings#connect_to_db? method!

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.5.0 (unreleased)
+  - Ensure rake tasks that don't need a DB connection can be run without one
   - Add "visibility" flag to GET /projects api endpoint
   - Ignore binary files in code search to prevent Error 500 (Stan Hu)
   - Upgrade gitlab_git to 7.2.23 to fix commit message mentions in first branch push

lib/gitlab/current_settings.rb

@@ -38,7 +38,7 @@ module Gitlab
                  true
                end
 
-      use_db && ActiveRecord::Base.connection.active? &&
+      use_db && ActiveRecord::Base.connected? &&
                 # The following condition is important: if a migrations adds a
                 # column to the application_settings table and a validation in
                 # the ApplicationSetting uses this new column we might end-up in