Make open user registration setting more prominent to admins

Background: By default, new GitLab self-hosted installations have new user sign-up enabled.

Problem: With the defaults settings, external or unwanted users can create new user accounts. The new accounts are added to the customer's true-up count and can be used to login to the instance if sign-up restrictions are not implemented.

Internal discussion thread

Proposal

• Adjust the ordering of the collapsible sections, so "Sign-up restrictions" is the first section.
• Present the "Sign-up restrictions" section expanded on viewing the settings page, so an admin can see all of the options.
• Describe this setting in our documentation.
• If we use helper text with checkboxes, add to the label for the "Sign-up enabled" option: