Ensure Auditor can see Billing page on gitlab.com

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.

Intended users

Support

Impact

Require less admin accounts on GitLab.com.

Implementation

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