Resolve "Provide an option to disable Two-Factor Authentication"

What does this MR do?

Add an application level setting for controlling the option to configure two factor authentication for all users. The three options for configuration include:

• 0 2FA is disabled & hidden for all users

• 1 no restrictions (default)

• 2 2FA is required for all users

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12813

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/35017

• https://gitlab.com/gitlab-org/gitlab-ce/issues/33508

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/26828

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides