Show billing page option to auditors in settings section in sidebar menu

Problem to solve

Iteration 3 of #351601 (closed). We need to make the Billing page option visible to auditors in the Settings section of sidebar menu.
Related discussion: #351601 (comment 1059124501)

Expected behaviour

Allow Auditor (in addition to Admin) to view Billing page option in Settings section of sidebar menu. This should be behind the auditor_billing_page_access flag.

Intended users

Support

Impact

Require less admin accounts on GitLab.com.

Implementation

Description	Issue	MR
Make the page available at /groups/<group-name>/-/billings for auditors	#351601 (closed)	!97430 (merged)
Show the page in read-only mode for auditors	#372964 (closed)	!101441 (merged)
Show the Settings -> Billing option for auditors	#378074 (closed)	!101949 (merged)