Allow password authentication to be disabled entirely

What does this MR do?

Implements the changes discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/37320:

• Rename the UI for "Sign-in enabled" to "Password authentication enabled for web interface"
• Add a new option for "Password authentication enabled for Git over HTTP(S)"

This development is sponsored by Siemens (/cc @bufferoverflow)

Are there points in the code the reviewer needs to double check?

For easier review:

• 00f17eb8 reverts the reverted changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13956
• e43e1c9e renames the password_authentication_enabled setting
• 439efc21 adds the password_authentication_enabled_for_git setting

I'll squash/reword the commits before merging.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12491 originally tried to fix some bugs when sign-in was disabled, but this broke a customer's workflow and was partly reverted in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13956. The proposed solution was to have two settings to support both use cases.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37320