Show Billing page to Auditor in read only mode

Problem to solve

Per our docs

Auditor users are able to have ... read-only access to all other resources, except the Admin Area [and Settings].

See &7469 for more details.

Expected behaviour

Allow Auditor (in addition to Admin) to view Billing page in read only mode. 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)