Improve clarity of the Rate Limits UI

What does this MR do?

Previously the admin interface for "User and IP Rate Limits" had inputs with duplicate labels. These inputs were only differentiated by their groupings on the page, which themselves were not clear. This improves the input labels, makes the checkbox labels bold (as they are the most important inputs in this section), and adds some horizontal rules to visually distinguish the groupings. These changes are intended to make this UI easier to understand, and match up better with the docs.

See the Rate Limit documentation for more details.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team