Show access as "Custom" in UI when custom role

What does this MR do and why?

• This is view only, there is no way to assign custom roles in the UI yet
• #384099 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Create a custom role and assign it to a user following the steps in !104322 (merged)
• View the members page for the group with the custom role
• Custom guests should show up as having a "Max role" of "Guest - custom"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.