Create Rake task to verify all encrypted data can be decrypted with instance's keys

We often see issues with errors such as OpenSSL::Cipher::CipherError (bad decrypt): in customer's HA environments. Our docs mention that /etc/gitlab/gitlab-secrets.json should be synced across all servers but it must be easy to miss. We should add a Rake task that will iterate through all encrypted data - project import data, CI variables, (and more?). If anything cannot be decrypted print out some info (type, id?). This should tell us reasonably well if any other instances in the cluster have differing keys.