Refactor: move user_login label to a separated file

MR on Jihu

What does this MR do and why?

We are going to allow users to login with their phone number on Jihu, then the label of user_login should be updated to Username, email or phone instead of Username or email on Jihu only.

We can not use translation to change that label because both _new_base_user_login_label.html.haml and _new_crowd.html.haml have _('Username or email') in the templates.

Proposal

1. move the user_login label to a separated haml file, so we can override it from Jihu side.

Screenshots or screen recordings

No UI updates

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @prajnamas