Feat: add prepend to devise failure

What does this MR do and why?

Related MR on Jihu.

Devise is using the translations located at config/locales/devise.en.yml, so we can't overwrite it in jh/locale/en/gitlab.po. To override the error messages, we need to extend devise_failure.rb to override the i18n_message method.

Proposal

1. add prepend_mod_with to gitlab/devise_failure.rb

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/cc @prajnamas