Exception when encrypting database fields using secrets containing multi-byte UTF characters

Description

During implementation of https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2639 we discovered that Encryptor / attr_encrypted requires 32 byte salt and 12 byte IV to encrypt data stored in the database.

In order to provide salt / IV we created following code:

AES256_GCM_OPTIONS = { 
  algorithm: 'aes-256-gcm',
  key: Settings.attr_encrypted_db_key_base_truncated,
  iv: Settings.attr_encrypted_db_key_base_truncated[0..11]
}.freeze

or

attr_encrypted :token,
               mode:      :per_attribute_iv,
               algorithm: 'aes-256-gcm',
               key:       Settings.attr_encrypted_db_key_base_truncated

in various places.

encrypted_db_key_base_truncated is implemented as

def attr_encrypted_db_key_base_truncated
  Gitlab::Application.secrets.db_key_base[0..31]
end

it means that if database key secret is using multi-byte UTF-8 characters encryption is going to fail, because

('❤' * 100)[0..31].bytesize
=> 96

/cc @nick.thomas @ifarkas @godfat @stanhu