Fixes needed when GitLab sign-in is not enabled

What does this MR do?

Fixes a number of bugs when GitLab sign-in is disabled.

When sign-in is disabled:

• skip password expiration checks
• prevent password reset requests
• don’t show Password tab in User Settings
• don’t allow login with username/password for Git over HTTP requests
• render 404 on requests to Profiles::PasswordsController

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

No.

Why was this MR needed?

We have sign-in disabled on our instance and use a CAS server for authentication, and we needed the fixes described above to ensure our users were not prompted to update expired passwords, didn't see a confusing Password tab, etc.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/25557 was originally intended to capture these problems.