Put Admin loggers in before_action for easier overriding in EE

What does this MR do?

Make it easier to override the set of loggers, by putting them in a separate method.

Also modify spec so it tests for links, instead of checking for regular text.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Needed by gitlab-org/gitlab-ee!2845