Conditional partial rendering for EE in application settings

What does this MR do?

Renders template partials in application settings conditionally for EE by utilizing render_if_exists helper. Existing code from application_settings/show.html.haml is moved to partials.

The corresponding MR for EE is: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7214/commits

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/44998

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