I18n support for cukes. Use i18n for labels login, register and 'forgot password'.

Related to [WP 5555 (multi-provider login forms)](https://www.openproject.org/work_packages/5555) where labels had been changed, which then had to be reverted since they broke a ton of tests.

This PR restores those label changes.

Following the merge of this PR changes to the cukes of to the following plugins are needed (PRs linked):

* [openproject-mobile_otp](https://github.com/finnlabs/openproject-mobile_otp/pull/21)
* [openproject-landing_page](https://github.com/finnlabs/openproject-landing_page/pull/23)
* [openproject-self_registration_by_mail](https://github.com/finnlabs/openproject-self_registration_by_mail/pull/15)